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The Bengali Calendar (Bengali: বঙ্গাব্দ Bônggabdo or বাংলা সন Bangla Shôn) is the sidereal solar used by the Bengali people in their calendar officially used in Bangladesh and West Bengal. The year begins on Pohela or Poila Boishakh, which falls on 14/15 April according to the tropical calendar in Bengal.
The origin of Bongabdo or Bangla Year is debated, with primarily two hypotheses, but the historicity of neither can be proved to date.
Mughal Emperor Jalaluddin Muhammad Akbar, the renowned grandson of Zahiruddin Muhammad Babur was the 3rd Mughal Emperor, introduced the Bengali Calendar. In order to make tax collection relatively easier, Akbar-e-Azam changed the practice of agricultural tax collection according to Hijri calendar and ordered an improvement of the calendar because the Hijri calendar, being a lunar calendar- did not agree with the harvest sessions and eventually the farmers faced severe difficulties in paying taxes out of season.
The regal astrologer of Emperor Akbar's reign, Aamir Fatehullah Siraji , was the one who in fact developed this calendar, after working out a research on the lunar Hijri and Solar calendars. The distinctive characteristic of the Bengali year was that, rather than being a lunar calendar, it was based on a union of the solar and lunar year. This was essentially a great promotion, as the solar and lunar years were formulated in very diverse systems.
Primarily this calendar was named as “Fasli San” and then Bongabdo or Bangla Year was launched on 10/11 March 1584, but was dated from 5th November 1556 or 963 Hijri. This was the day that Akbar defeated Himu in the clash of Panipat to ascend the throne.
Akbar-e-Azam’s ordered to resolve all dues on the last day of Choitro. The next day was the first day of the New Year (Bengali New Year), the day for a new opening; landlords used to allocate sweets among their tenants, and businessmen would commence a “Halkhata” (new financial records book) and lock their old ones. Vendors used to provoke their consumers to allocate sweets and renew their business relationship with them. There were fairs and festivities all over and gradually Poyela Boishakh became a day of celebration.
The Bengali calendar consists of 6 seasons, with two months comprising each season. Beginning from Pohela Boishakh, they are Grishsho (গ্রীষ্ম) or Summer; Bôrsha (বর্ষা) or Rainy/Monsoon season; Shôrot (শরৎ) or Autumn; Hemonto (হেমন্ত) or the Dry season; Šit (শীত) or Winter; and Bôshonto (বসন্ত) or Spring. But as the traditional calendar used in India is sidereal, it does not correspond to the actual tropical movement of the earth. Hence, after some centuries the months will shift far away from the actual seasons. But the reformed tropical version of the calendar used in Bangladesh will continue to maintain the seasons as mentioned above.
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The names of the twelve months of the Bengali calendar are based on the names of the নক্ষত্র nokkhotro (lunar mansions): locations of the moon with respect to particular stars during the lunar cycle. These names were derived from the Surya Siddhanta, an ancient Indian book on Astronomy. The names of the months are:
- বৈশাখ Boishakh after the star, বিশাখা Bishakha
- জ্যৈষ্ঠ Joishţho after the star, জ্যেষ্ঠা Jeshţha
- আষাঢ় Ashaŗh after the star, উত্তরাষাঢ়া Uttorashaŗha
- শ্রাবণ Srabon after the star, শ্রবণা Srobona
- ভাদ্র Bhadro after the star, পূর্বভাদ্রপদ Purbobhadropôd
- আশ্বিন Ashshin after the star, অশ্বিনী Ôshshini
- কার্তিক Kartik after the star, কৃত্তিকা Krittika
- অগ্রহায়ণ(মার্গশীর্ষ) Ôgrohaeon after the star, মৃগশিরা Mrigoshira
- পৌষ Poush after the star, পুষ্যা Pushsha
- মাঘ Magh after the star মঘা Môgha
- ফাল্গুন Falgun after the star, উত্তরফাল্গুনী Uttorfalguni
- চৈত্র Choitro after the star, চিত্রা Chitra
Names and approximate lengths of Bengali months :
|No.||Name||Bengali||Sanskrit||Days (Traditional Hindu sidereal solar calendar)||Days (Revised version as used in Bangladesh)|
|1||Boishakh||বৈশাখ||वैशाख||30 / 31||31|
|2||Joishtho||জ্যৈষ্ঠ||ज्येष्ठ||31 / 32||31|
|3||Ashadh||আষাঢ়||आषाढ||31 / 32||31|
|4||Srabon||শ্রাবণ||श्रावण||31 / 32||31|
|5||Bhadro||ভাদ্র||भाद्रपद,भाद्र,प्रोष्ठपद||31 / 32||31|
|6||Ashvin||আশ্বিন||आश्विन,अश्वयुज||31 / 30||30|
|7||Kartik||কার্তিক||कार्तिक||29 / 30||30|
|8||Ogrohayon||অগ্রহায়ণ||अग्रहायण,मार्गशीर्ष||29 / 30||30|
|9||Poush||পৌষ||पौष||29 / 30||30|
|10||Magh||মাঘ||माघ||29 / 30||30|
|11||Falgun||ফাল্গুন||फाल्गुन||29 / 30||30 (31 days in leap year)|
|12||Choitro||চৈত্র||चैत्र||30 / 31||30|
The Bengali Calendar incorporates the seven-day week as used by many other calendars. Also like other calendars, the names of the days of the week in the Bengali Calendar are based on celestial objects, or নবগ্রহ nôbogroho.
- Monday: সোমবার Shombar after সোম Shom (a Lord Shiva or Moon)
- Tuesday: মঙ্গলবার Monggolbar after মঙ্গল Monggol (planet Mars)
- Wednesday: বুধবার Budhbar after বুধ Budh (planet Mercury)
- Thursday: বৃহস্পতিবার Brihoshpotibar after বৃহস্পতি Brihoshpoti (planet Jupiter)
- Friday: শুক্রবার Shukrobar after শুক্র Shukro (planet Venus)
- Saturday: শনিবার Shonibar after শনি Shoni (planet Saturn)
- Sunday: রবিবার Robibar after রবি Robi (Sun)
In the Bengali calendar, the day begins and ends at sunrise, unlike in the Gregorian calendar, where the day starts at midnight.
Revised Bengali calendar 
The length of a year in the Bengali calendar, as in the Gregorian calendar, is counted as 365 days. However, the actual time taken by the earth in its revolution around the sun is 365 days 5 hours 48 minutes and 47 seconds. To make up this discrepancy, the Gregorian calendar adds an extra day, to make a leap year, to the month of February every fourth year (except in years divisible by 100 but not by 400). The Bengali calendar, which was based on astronomical calculations, did not make this extra leap year adjustment. Bengali months, too, were of different lengths. To counter this discrepancy, and to make the Bengali calendar more precise, the following recommendations of the Bangla Academy are followed:
- The first five months of the year from Boishakh to Bhadro will consist of 31 days each.
- The remaining seven months of the year from Ashshin to Choitro will consist of 30 days each.
- In every leap year of the Gregorian calendar, an additional day will be added in the month of Falgun, which is just 14 days after 29 February. (Modified without material change).
The revised calendar was officially adopted in Bangladesh in 1987. However, it is not followed in the neighbouring state of West Bengal, India, where the traditional calendar continues to be followed due to the deep bond of Hindu culture with the Bengali calendar. Hindu religious festivals are celebrated based on a particular lunar day and Bengali calendar combination.
Revised and non-revised versions 
The first of Boishakh, Pôhela Boishakh, is the Bengali New Year's Day. In West Bengal, it is celebrated on 14/15 April. However, since the calendar was revised in Bangladesh the new year now always falls on 14 April. Hindus in Bangladesh refer to the original Bengali calendar in religious practices.
The Bengalis in West Bengal, and the Hindu Bengalis in Bangladesh follow a sidereal solar calendar unlike the tropical year solar calendars, such as the revised Bengali and Gregorian Calendars. The mathematical difference between the sidereal and the tropical calendars accounts for the difference of starting the new year in Bangladesh and West Bengal, India. Because of this the length of the months are also not fixed in the Bengali sidereal calendar, but rather are based on the true movement of the sun.
Leap year 
Although the sidereal solar calendar is followed in West Bengal, India, the number of days in the months are determined by the true motion of the Sun through the zodiac. In this calendar, seven is subtracted from the year, and the result is divided by 39. If after the division the remainder (= (year - 7) / 39) is zero or is evenly divisible by 4, the year is then designated as a leap year and contains 366 days, with the last month, Choitro, taking 31 days. There are 10 leap years in every 39 years, although an extraordinary revision may be required over a long time.
According to the new calendar system in Bangladesh, Falgun (which begins mid-February) has 31 days every four years. To keep pace with the Gregorian calendar, the Bengali leap years are those whose corresponding Gregorian calendar year is counted as a leap year. For example, Falgun 1410 was considered a Bengali leap month, as it fell during the Gregorian leap month of February 2004.
The usage and popularity of the Bengali calendar in eastern South Asia is partly due to its adaptation to the unique seasonal patterns of the region. Eastern South Asia has a climate that is best divided into six seasons, including the monsoon or rainy season and the dry season in addition to spring, summer, fall, and winter.
In everyday use, the Bengali Calendar has been largely replaced by the Gregorian Calendar in Bengali-speaking regions, although it is still essential for marking holidays specific to Bengali culture (e.g. Pôhela Boishakh, Durga Puja, etc.), and for marking the seasons of the year. The Bengali calendar is recognized by the government of Bangladesh, whose offices date all their correspondence with the Bengali date as well as the Gregorian one. Almost every Bengali- and English-language newspaper in Bangladesh and West Bengal prints the day's date according to the Bengali Calendar alongside the corresponding date of the Gregorian Calendar. Many newspapers in Bangladesh also add a third date, following the Islamic Hijri Calendar. Thus, it is quite common in Bangladesh to find the date written three times (e.g. "15 Falgun 1412, 17 Muharram 1427, 27 February 2006") under the newspaper title.
Related calendars 
The Bengali calendar is related to the Hindu solar calendar, which is itself based on the Surya Siddhanta. The Hindu solar calendar also starts in mid-April, and the first day of the calendar is celebrated as the traditional New Year in Mithila, Assam, Kerala, Manipur, Nepal, Orissa, Punjab, Tamil Nadu, and Tripura in addition to Indian state of West Bengal and Bangladesh. Nepal, Thailand and Sri Lanka also celebrate new year around the same time (12–15 April). This is also known as Mesha Shong-kronti.
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