Why Does Diwali's Date Change Every Year?

Diwali always falls on the same lunar date: the Amas (new moon) of Aso month. What changes is only the Gregorian date, because the lunar year is about 11 days shorter than the solar year.

To keep the two aligned, an extra month (adhik maas) is inserted roughly every three years. That is why Diwali drifts between mid-October and mid-November.

The Vikram Samvat new year, Bestu Varas, begins on Kartak Sud 1. Usually that is the very next day, but when Amas is still running at the next sunrise, a gap day called the dhoko falls in between, as in 2026: Diwali on 8 November and Bestu Varas on 10 November.