Hindu religious festivals are based on Vikram Samvat. Notwithstanding the Purnimanta scheme of months that is in use in North India, the New year in Vikram Samvat starts from the first day of Chaitra Shukla Paksha.
In Gujarat, the next day after Diwali is celebrated as the first day of the Vikram Samvat calendar which is the first day of the month Kartik.