- In Jewish law, a calendar day is defined as running from "evening" to "evening." This is based on the repetition of the phrase "... and there was evening, and there was morning ..."—evening preceding morning—in the account of creation in Genesis.
- Additionally, Jewish law requires certain activities to be undertaken "during the day"—or at a certain time during the day—while other activities are to be undertaken "at night"—or at a certain time during the night.
For either purpose, the status of the twilight hours just after sunset or just before sunrise is ambiguous. Judaism provides its own definitions for this period; at the same time, various rabbinic authorities differ on just how those definitions are to be applied for different purposes.
General Approach 
The Talmud often states calculations of these zmanim in terms of the time it takes to walk some distance, stated in "mils". Most authorities reckon the time it takes to walk one mil as being 18 minutes, though there are opinions of up to 24 minutes. Many authorities, indeed, hold such calculations to be absolute: if nightfall follows sundown by "the amount of time it takes to walk four mils," for example, that means "72 minutes after sundown" in all places on all dates. Other authorities, especially those living in higher latitudes, noted that the darkness of the sky 72 minutes after sundown (for example) can vary substantially from place to place, and from date to date. They therefore hold that "72 minutes after sundown" actually refers to how dark the sky is 72 minutes after sundown in Jerusalem on an equinox. Then that degree of darkness is reckoned as being true when the sun has fallen a certain number of degrees below the horizon (for example, 7°5′ below the horizon), and that becomes the actual standard used for all places and all dates.
One calendar day ends, and the next day begins, during the evening. The Talmud states there is an uncertainty as to whether the day ends at sunset or nightfall, so the time in between has a status of doubt. The Talmud in Pesachim states there are four mil between sunset and nightfall. However, the Talmud in Tractate Shabbat states that there are just three-quarters of a mil between sunset and nightfall.
- The Geonim and the Vilna Gaon (GR"A) explain that the second statement is correct.
- Rabbeinu Tam and many other Rishonim say rather that there are two "sunsets," the first of which [actual sunset] is four mil before nightfall, and the second of which is just three-quarters of a mil before.
According to the first opinion, nightfall is 13½-18 minutes after sundown (or its equivalent, sun falls 3-4° below the horizon). According to the second opinion, nightfall occurs exactly 72 minutes after sundown (or its equivalent, when the sun has fallen anywhere from 6.45 to 16.1 degrees below the horizon).
In practice, when the question is whether it is too late to do a something that must be done by day, the practice is to follow one of the earlier opinions. When the question is whether it is too early to do something that must be done by night, the practice is to follow one of the later opinions. For calculating the end of Shabbat or major holidays, the desire not to end Shabbat early, along with the Talmud's description of the end of Shabbat in terms of the appearance of stars in the sky, leads to the use of the later opinions. (See End of Shabbat below.)
There is also a difference of opinion as to when one may begin to do activities than must be done during the day:
- Daybreak, when some light is visible, or
- Sunrise, when the ball of the sun rises above the horizon.
The Talmud in Pesachim (see above) holds symmetrically that the time between daybreak and sunrise is also the time in which one can walk four mils. For morning calculations, daybreak is normally held to be when the sun is 16.1° below the horizon, or a fixed 72 minutes or 90 minutes before sunrise.
- A little after daybreak is a time known as misheyakir, "when one can recognize [another person four cubits away]." This calculation is used as the earliest time to wear tzitzit, and in parallel, tefillin, because this is when the blue thread of the tzitzit can be distinguished from the white threads. Misheyakir is always calculated relative to season and place, and occurs when the sun is either 10.2-11.5 degrees below the horizon.
Seasonal hours 
For almost all halakhic purposes, each day is divided into twelve equal "hours." There are two major opinions:
- The Magen Avraham holds that because one may do "daytime" activities between daybreak and nightfall, one calculates the day from daybreak to nightfall, and divides that period into twelve parts. (Magen Avraham holds daybreak as 72 minutes before sunrise, and nightfall as 72 minutes after sunset, for this purpose.)
- The Vilna Gaon holds that although "daytime" activities can start as early as daybreak and end as late as nightfall, their proper time lechatchila (ab initio) is from sunrise to sunset, so one calculates the day from sunrise to sunset and divides that period into twelve parts.
The result is that "Magen Avraham times" are earlier in the morning than "Vilna Gaon times"; in practice there are communities that follow each of those standards. For times in the afternoon, the Vilna Gaon's times are earlier, and are almost universally followed.
These are called "seasonal" or "variable" hours because they depend on the length of time between sunrise (or daybreak) and sunset (or nightfall), and those vary through the year. Near New York, for example, a "seasonal hour" based on the Vilna Gaon's calculations lasts around 45 minutes near winter solstice, around 60 minutes near the equinoxes, and around 75 minutes near summer solstice.
Daybreak (עֲלוֹת הַשַּׁחַר, Alot Hashachar) refers to when the first rays of light are visible in the morning.
- If one has not recited the evening Shema by this time, and the omission was not due to negligence, one can still recite it now, up to sunrise, though one may not say Hashkiveinu or Baruch Hashem L'Olam.
- If one has prayed Shacharit after this time, one has fulfilled his obligation ex post facto. Furthermore, most mitzvot that must be performed during the day (such as the Four Species or Hallel) may be done after this time, at least ex post facto.
- One does not use tallis and tefillin before misheyakir (משיכיר).
Sunrise (נֵץ הַחַמָּה, Netz Hachamah) refers to when the ball of the sun rises above the horizon. Technically, it is preferable to pray the morning Shema just before this time and begin the Amidah just afterwards, although this is rarely done nowadays. Most mitzvot that must be performed during the day (such as the Four Species or Hallel) should be done after this time ab initio.
Sof Zman Kriyat Shema 
Sof Zman Kriyat Shema (סוֹף זְמַן קְרִיאַת שְׁמַע) means "end of the time to say the [morning] Shema." This is three halachic hours into the day. These hours are variable/seasonal hours and refer to one twelfth of the time between daybreak and nightfall (according to the Magen Avraham) or one twelfth of the time between sunrise and sunset (according to the Vilna Gaon). The latter time is always thirty-six minutes after the former time.
Sof Zman Tefilah 
Sof Zman Tefilah (סוֹף זְמַן תְּפִלָּה) means "end of the time to say the Shacharit Amidah." This is four (variable} hours into the day. The above comments apply here also, with the later time twenty-four minutes after the earlier time. However, since the Amidah is only rabbinically required (unlike the Shema which is Scriptually mandated) it is common to rely on the later time, and few calendars publish the earlier time.
Midday (חֲצוֹת הַיּוֹם, Chatzot Hayom or just Chatzot) means the midpoint between sunrise and sunset, or equivalently between daybreak and sundown. The absolute latest time for the Shacharit Amidah, ex post facto, is this time. On the Shabbat and on holidays, one is supposed to eat before this time (except on Rosh Hashanah). On Tish'a Ba'av one may sit on a chair at this time, and those who fast on Erev Rosh Hashanah usually eat at this time.
Mincha Gedolah 
Minchah Gedolah' (מִנְחָה גְּדוֹלָה, literally the greater Minchah), one-half variable hour after midday, is the earliest time to recite Minchah, although one should try, if possible, to wait until Minchah Ketanah. On Yom Kippur, the congregation must begin Mussaf by this time, because otherwise they would be required to pray the more frequent prayer (Mincha) first.
Mincha Ketanah 
Minchah Ketanah (מִנְחָה קְטַנָּה, literally the lesser Minchah), two and one-half variable hours before sunset, is the preferable earliest time to recite Minchah.
Plag Hamincha 
Plag Hamincha (פְּלַג הַמִּנְחָה, literally half of the Minchah) is the midpoint between Minchah Ketanah and sunset, i.e. one and one-quarter variable hours before sunset. If one prayed Minchah before this time, one may recite Maariv afterwards (at the conclusion of the Sabbath, this may only be done under extenuating circumstances). Otherwise, one must wait until sunset, unless one is praying as a congregation.
Sunset (שְׁקִיעַת הַחַמָּה, Shkiyat Hachamah) is the time at which the ball of the sun falls below the horizon. The new day begins at this point for almost all purposes. For example, the calendar date changes, and a meal eaten now includes the new day's additions in Grace after Meals (except at the conclusion of the Sabbath). Mitzvot that must be performed during the day may no longer be performed ab initio. Minchah should not be delayed past now. Maariv may be recited now, although many wait until after nightfall.
Bein Hashemashot 
Bein Hashemashot (בֵּין הַשְּׁמָשׁוֹת, literally between the suns) is the period between sunset and nightfall, and is considered a time of questionable status. On the Sabbath, festivals, and fast days the stringencies of both the previous and following days apply. For example, if the fast of Tish'a Ba'av immediately follows the Sabbath, the intervening Bein Hashemashot is forbidden in eating, drinking, and working. However, there are occasional leniencies.
Nightfall (צֵאת הַכּוֹֹכָבִים, Tzet Hakochavim) is described in detail above. After nightfall, it is considered definitely the following day. All restrictions of the previous day go away, and any Mitzvot that must be performed at night (such as the evening Shema, the Seder, or Bedikas Chametz may be performed.
End of Shabbat 
מוצאי שבת, the End of Shabbat (or equivalently, the end of a festival or fast), is described in the Talmud as a time when "three medium stars are [visible] in the sky." There is much discussion of what this means, and because of a strong desire not to end Shabbat "too" early, it tends to be construed strictly in practice. There are three widely observed practices, all of which have support in the halachic literature:
- Appearance of three medium-sized stars in the sky (sun 7°5' below the horizon, or 42 minutes after sundown), as in the Talmud. This is normative practice in Conservative Judaism. In Orthodox Judaism, this position is used widely for the end of rabbinical fasts, but less frequently for the end of Shabbat or biblical festivals.
- Appearance of three small stars widely spaced in the sky (sun 8.5°-8.75° below the horizon): common practice in much of Orthodox Judaism 
- "50 minutes after sundown" is actually a variant of this position. Rabbi Moshe Feinstein ruled this way because most people cannot easily calculate when "8.5° after sundown" will occur, and 8.5° takes 50 minutes at its longest, near the summer solstice, at the latitude of much of the United States.
- 72 minutes after sundown ("opinion of Rabbeinu Tam"): equivalent to other definitions of nightfall, and safe according to all opinions. Common practice in Chasidic and other Charedi communities
Midnight (חֲצוֹת הַלַּילָה, Chatzot Halaylah or just Chatzot) is the midpoint between nightfall and daybreak, or equivalently between sunset and sunrise. The evening Shema should be recited by now, and the Afikoman on Passover should be eaten by this time. The Talmud in Berachot rules that all "night" mitzvot should be performed by Chatzot, at least ab initio, in case the person would otherwise fall asleep and then fail to perform the mitzvot. Some rise at this time and recite Tikkun Chatzot, a series of supplications for the rebuilding of the Temple.
Other Zmanim 
The Mussaf prayer should preferably be recited before seven variable hours, on days it is recited.
See also 
- Genesis 1:5, Gen 1:8, Gen 1:13, Gen 1:19, Gen 1:23, Gen 1:31 and Gen 2.2.
- "MyZmanim Sources". MyZmanim.com. Retrieved 4 December 2012.
- "Computation by Degrees Explained". MyZmanim.com. Retrieved 11 December 2012.
- Babylonian Talmud, Pesachim 94a)
- "Zmanim Terms". Kehillat Israel. Retrieved 4 December 2012.
- Based on a 22½ minute mil
- "Zmanim for Highland Park (NJ)". MyZmanim.com. Retrieved 11 December 2012.
- "More on Plag Hamincha". Chabad. Retrieved 4 December 2012.
- "Starry Starry Night". Ohr Sameyach. Retrieved 11 December 2012.
- Shulchan Aruch Orach Hayyim 261, and Mishnah Berurah note 23 there. (link)