By the way, today is also Sankranti. https://twitter.com/RavikiranRao/status/1249913294485245952

Not everyone celebrated Sauramana Yugadi yesterday, but it is Sauramana Yugadi that acts as the synchronization point for all the Hindu calendars in existence today.

The Chandramana Yugadi, (also Gudi Padwa) is always the first day after the new moon that comes immediately before Sauramana Yugadi. That is the first day of the Hindu calendar for most of India.

I think Bihar and a few other places use Holi as New Year's day. That is of course the day after the full moon that comes immediately before Chandramana Yugadi.

The rest of the calendar followed by Hindu is of course lunar, and is determined by the lunar months and phases of the moon. But the lunar year is 354 days, so once in a while, an extra month needs to be added.

The way it is added is via the concept of adhika maasa. The Sankranti that falls on the 14th/15th day of each month is used for that as well. If there are two new moons that occur between 2 Sankrantis, then the day after the first new moon is the start of the adhika maasa.

As to why these Sankrantis are used as the synchronization points - the obvious reason is that when the calendar was adopted, these days corresponded to the transition of the sun to the next rashi. Jan 14 and July 14 were the solstices and April and October 14 were equinoxes.

Due to the precession of the equinoxes, the actual equinoxes have shifted by around 24 days. The equinoxes shift by one day every 71 years, so, the calendar must have been adopted around 1700 years ago.

The calendar that is followed in most of India is the Vikram Samvat, which tells us that it is year 2077. I am guessing that the actual date when the calendar was adopted and the start of the calendar must be different.

(Just as the Christian calendar was actually adopted in around AD 400, IIRC)

Any corrections to the above welcome. I am not the expert, btw.