The Bengali calendar is tied to the Indian solar calendar, based on the Surya Siddhanta. The first day of the Bengali year coincides with the mid-April new year in Mithila, Assam, Burma, Cambodia, Kerala, Manipur, Nepal, Odisha, Sri Lanka, Tamil Nadu and Thailand. All Bengali Hindus follow the traditional date of 15 April as Poila Boishakh. The length of a year is counted as 365 days, as in the Gregorian calendar. 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). 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 Bôishakh to Bhadrô will consist of 31 days each.
The remaining seven months of the year from Ashwin to Chôitrô 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 14 days after 29 February).
The first day of the New Year of the Indian solar calendar and all derived calendars including Bengali calendar is the first day of the new year, and historically the day has been seen across the subcontinent as the day for a new opening and celebrated accordingly. In Bengal landlords used to allocate sweets among their tenants, and business people commenced a “Halkhata” (new financial records book) and locked 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.