- For the anime, see Dog Days (anime)
The phrase dog days refers to the sultry days of summer. In the Northern Hemisphere, the dog days of summer are most commonly experienced in the months of July and August, which typically observe the warmest summer temperatures. In the Southern Hemisphere, they typically occur in January and February, in the midst of the austral summer.
The name 
The Romans referred to the dog days as diēs caniculārēs and associated the hot weather with the star Sirius. They considered Sirius to be the "Dog Star" because it is the brightest star in the constellation Canis Major (Large Dog). Sirius is also the brightest star in the night sky. The term "Dog Days" was used earlier by the Greeks (see, e.g., Aristotle's Physics, 199a2).
The Dog Days originally were the days when Sirius rose just before or at the same time as sunrise (heliacal rising), which is no longer true, owing to precession of the equinoxes. The Romans sacrificed a brown dog at the beginning of the Dog Days to appease the rage of Sirius, believing that the star was the cause of the hot, sultry weather.
Dog Days were popularly believed to be an evil time "the Sea boiled, the Wine turned sour, Dogs grew mad, and all other creatures became languid; causing to man, among other diseases, burning fevers, hysterics, and phrensies." according to Brady’s Clavis Calendaria, 1813.
The dates 
In Ancient Rome, the Dog Days ran from July 24th through August 24th, or, alternatively, from July 23 through August 23rd. In many European cultures (German, French, Italian) this period is still said to be the time of the Dog Days.
The Old Farmer's Almanac lists the traditional period of the Dog Days as the 40 days beginning July 3rd and ending August 11th, coinciding with the ancient heliacal (at sunrise) rising of the Dog Star, Sirius. These are the days of the year with the least rainfall in the Northern Hemisphere.
According to the 1552 edition of the The Book of Common Prayer, the "Dog Daies" begin July 6th and end August 17th. But this edition, the 2nd book of Edward VI, was never used extensively nor adopted by the Convocation of the Church of England. The lectionary of 1559 edition of the Book of Common Prayer indicates: "Naonae. Dog days begin" with the readings for July 7th and end August 18th. But this is noted as a misprint and the readings for September 5th indicate: "Naonae. Dog days end". This corresponds very closely to the lectionary of the 1611 edition of the King James Bible (also called the Authorized version of the Bible) which indicates the Dog Days beginning on July 6th and ending on September 5th. A recent reprint of the 1662 edition of the Book of Common Prayer contains no reference to the Dog Days.
Please note: Due to introduction of the modern Gregorian Calendar, 11 days must be added to each of the 16th and 17th Century dates referenced above for them to correlate correctly with modern-day dates as concerns astronomical observations and climate. First adopted by Southern European Catholic countries in the 16th Century, the Gregorian Calendar was not used in England or its New World colonies until 1752. This modern calendar accurately calculates the astronomical length of one year (the exact time it takes the earth to orbit the sun) to be 365.2425 days. This corrected the approximated 365 and 1/4 day year length of the previously used Julian Calendar introduced in 46 B.C. Because the length of each Julian Calendar year was 11 minutes 48 seconds too long, over the centuries, seasonal changes gradually occurred on earlier and earlier dates. The Gregorian Calendar uses a formula to reduce from 100 to 97 the number of leap years (extra days) in a 400-year period thus cutting the average calculated length of a year to its actual duration. By 1752 when the Gregorian Calendar was adopted by England, the Julian Calendar had fallen behind by 11 days total since 46 B.C. Upon implementation of the Gregorian Calendar, those days were added back by jumping overnight from September 2nd to September 14th, 1752, thus catching the calendar up with the seasons.
The Book of Common Prayer would have provided the official liturgical calendar for Jamestown, Virginia, from 1607 so it may be assumed that the Dog Days likely have been known in the New World at least since that time.
Other references 
For the ancient Egyptians, Sirius appeared just before the season of the Nile's flooding, so they used the star as a "watchdog" for that event. Since its rising also coincided with a time of extreme heat, the connection with hot, sultry weather was made for all time: "Dog Days bright and clear / indicate a happy year. / But when accompanied by rain, / for better times our hopes are vain."
The phrase is mentioned in the short story "The Bar Sinister" by Richard Harding Davis. The main character, who is a street dog, explains "but when the hot days come, I think they might remember that those are the dog days, and leave a little water outside in a trough, like they do for the horses."
The Prologue of Tuck Everlasting, set in the first week of August, says: "These are strange and breathless days, the dog days, when people are led to do things they are sure to be sorry for after."
In recent years, the phrase "Dog Days" or "Dog Days of Summer" have also found new meanings. The term has frequently been used in reference to the American stock market(s). Typically, summer is a very slow time for the stock market, and additionally, poorly performing stocks with little future potential are frequently known as "dogs."
A casual survey will usually find that many people believe the phrase is in reference to the conspicuous laziness of domesticated dogs (who are in danger of overheating with too much exercise) during the hottest days of the summer. When speaking of "Dog Days" there seems to be a connotation of lying or "dogging" around, or being "dog tired" on these hot and humid days. A similar myth asserts that the time is so-named because rabid dogs are supposed to be the most common then. Although these meanings have nothing to do with the original source of the phrase, they may have been attached to the phrase in recent years due to common usage or misunderstanding of the origin of the phrase.
Oh! But he was a tight-fisted hand at the grind-stone, Scrooge! A squeezing, wrenching, grasping, scraping, clutching, covetous, old sinner! Hard and sharp as flint, from which no steel had ever struck out generous fire; secret, and self-contained, and solitary as an oyster. The cold within him froze his old features, nipped his pointed nose, shrivelled his cheek, stiffened his gait; made his eyes red, his thin lips blue; and spoke out shrewdly in his grating voice. A frosty rime was on his head, and on his eyebrows, and his wiry chin. He carried his own low temperature always about with him; he iced his office in the dogdays; and didn't thaw it one degree at Christmas.
Poet J. M. Synge also wrote, "Seven dog-days we let pass, naming Queens in Glenmacnass", in the poem "Queens".
The phrase is used as the title of a sonnet by Australian poet, Howard Firkin.
The 1975 Sidney Lumet film Dog Day Afternoon takes its name from the phrase, as does the 2012 film "Diary of a Wimpy Kid : Dog Days", about the activities of a teenage boy during his summer school holidays.
In the Batman: Arkham City videogame, the serial killer Calendar Man tells stories to Batman about murders he has committed on a major holiday for each month. For the month of August, he chose the Feast Day of St. Roch (which he admitted to be an obscure holiday) as the basis for a dog-related murder, the best way to "celebrate the dog days of summer."
See also 
|Look up dog days in Wiktionary, the free dictionary.|
- Brady, J: "Clavis Calendaria", vol. II, page 89. Nichols, Son, and Bentley, 1815.
- Booty, John (1976). The Book of Common Prayer, The Elizabethan Prayer Book. Folger Shakespeare Library, Washington DC: Folger Books. pp. 42–44. ISBN 918016-58-4 Check