Here below is some info on the history of Bengali New Year.
Today is the Bengali New Year, known in Bangla as Pohela Boishakh. A number of Indian calendars, of course, have their new year around now: the Punjabi, Assamese and Tamil, to name a few. The modern Bengali calendar though is unique amongst these, given that it was introduced by the Mughal Empire.
More than two decades into his rule, Emperor Akbar, third in the Mughal line, had set up, what was at the time, the most powerful empire on Earth. Secure in his power, the emperor’s attention shifted to the more intellectual side of things: religion, philosophy and the arts. Amartya Sen’s book, The Argumentative Indian, mentions how Akbar's interest in various religions led him to dabble in the calendars of various faiths as well. As a result, as Sen put it, he invented “a combined calendar which paralleled his interest in floating a combined religion, the Din-e-Ilahi”. This calendar, modestly titled the Tarikh-e-Ilahi, calendar of God, was introduced in the year 1584 AD.
Many historians believe, however, that Akbar chose the calendar not out of any interest in theology but in response to a much higher power: taxes. In their compendium, the Historical Dictionary of the Bengalis, Kunal Chakrabarti and Shubhra Chakrabarti write that earlier, the Mughal Empire was having trouble collecting land revenue since they followed the Islamic Hijri calendar. Given that the Islamic calendar is lunar, it did not coincide with the seasons, leading to much confusion.
Akbar therefore asked his royal astronomer to devise a new calendar which merged together the Islamic calendar, the historical Bengali calendar (based upon a Sanskrit astronomy text, the Surya Sidhant) and Akbar’s own date of coronation. The last point might sound a bit pompous but, as anyone who’s seen Mughal-e-Azam would attest to, Mr Akbar did have a bit of an ego problem.
The new calendar that was devised was a bit complex. Its first year, just like the Islamic calendar, was the date of the Hijra, Prophet Mohammed’s emigration from Mecca to Medina. From this year 1 to Akbar’s coronation (in 1556 AD), the calendar ticks off the years as a lunar calendar. Up till here, the Tarikh-e-Ilahi and the Islamic calendar are in step: for both, Akbar's coronation occurs in the year 963. From this annum onwards though, things change – after all, it’s a big year: the Emperor's coronation. From the coronation onwards, the years start to tick off as per the old traditional Bengali calendar, which was a solar one, and solves the problem of mismatched seasons.
Bengali year 1422
The “formula”, as it were, for calculating the Bengali year, therefore is: Islamic year at Akbar’s crowning (963) + current Gregorian solar year (2015) - Gregorian solar year at Akbar’s crowning (1556).
This gives us 1422, which, voila, is the Bengali year which starts today.
The Tarikh-e-Ilahi was introduced for the entire Mughal Empire and, like the Din-e-Ilahi, it really didn’t last much beyond Akbar’s lifetime. The one exception was in Bengal, where it became integral to both agriculture as well as the Hindu religion. As a result, it is interesting to note that when, say, Durga Pujo dates will be calculated for the Bengali year 1422, the two events referenced (unknown to most Bengalis themselves) will be the migration of Prophet Mohammad from Mecca to Medina and Akbar’s coronation.