Calendar reform in India
India's national calendar was reviewed in the 1950s by a Calendar Reform Committee, which recommended a standard civil calendar based on the Saka era.
What it is
The committee's report, which included a sidereal reference for the astronomical almanac, was published in 1955. The national Saka calendar was adopted in 1957 for official use, and the Lahiri ayanamsa is linked to the committee's astronomical work.
How it is worked out or read
The national calendar is used for official dates, while many festivals and panchangs continue to follow traditional rules. The two calendars can give different dates for the same festival.
Where it appears on this site
The ayanamsa is discussed in the Lahiri article. The Saka era is in the era article.
The problem in 1952
At independence, India had no single calendar for civil and religious purposes. Dozens of regional calendars were in use, solar and luni-solar, with different year beginnings, different month systems and different calculation methods. Some almanacs used modern astronomical data; many still used the parameters of the old siddhantas, which had drifted from the sky. Festival dates could differ between almanacs, and the same festival could be kept on different days in different places.
In 1952 the Government of India set up a Calendar Reform Committee, chaired by the physicist Meghnad Saha, to study the situation and recommend a unified calendar.
What the committee found and proposed
The committee surveyed the calendars in use and reported that about thirty were being followed in different parts of the country. Its report, published in 1955, made two kinds of recommendation. For civil use, it proposed a national calendar based on the Saka era, with fixed month lengths aligned to the tropical year. For religious use, it proposed that almanac makers compute positions from modern astronomy rather than siddhanta tables, and that they adopt a standard ayanamsa so that sidereal positions would be uniform across the country.
The Indian National Calendar
The national calendar was adopted from 22 March 1957, which was Chaitra 1 of the Saka year 1879. Its year begins on 22 March, or on 21 March in Gregorian leap years. Chaitra has 30 days, or 31 in a leap year; the next five months, Vaishakha to Bhadra, have 31 days each; and the remaining six have 30. The calendar appears alongside the Gregorian date in government gazettes, on All India Radio announcements and in official publications, though in daily life most people still use the Gregorian calendar and their regional almanac.
The ayanamsa recommendation
The committee's ayanamsa recommendation, associated with N. C. Lahiri, who served on it, is the origin of the Lahiri or Chitrapaksha ayanamsa that is now the most widely used in India and is the default on this site. It places the star Chitra, Spica, at the middle of the sidereal sign Kanya, at 180 degrees from the start of Mesha. An official almanac, the Rashtriya Panchang, is prepared each year by the Positional Astronomy Centre in Kolkata on this basis.
What changed and what did not
The reform did not replace regional calendars, which continue to govern festivals and family observances. What it did was establish a common astronomical standard that many almanac makers adopted, and so reduce the differences caused by outdated parameters. Today, when two almanacs disagree, the cause is more often a difference of tradition or sunrise convention than an error in the underlying astronomy.