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which is the first month of the national calender of india
The national calendar based on the Saka Era, with Chaitra as its first month and a normal year of 365 days was adopted from 22 March 1957.
how many days in each month of the national calendar of India
The months in the first half of the year all have 31 days, to take into account the slower movement of the sun across the ecliptic at this time.
How are the years in the national calendar of India counted
Years are counted in the Saka era, which starts its year 0 in the year 78 of the Common Era. To determine leap years, add 78 to the Saka year.
Where are the names of the months in the National calendar of India derived from
Months' names are derived from older, Hindu lunisolar calendars, so variations in spelling exist, and there is a possible source of confusion on what calendar a date belongs to.

Question: Why the decision to adopt the calendar
Self-contained rewrite: Why the decision to adopt the national calendar in India