Short answers, pulled from the story.
The three national public holidays in India are Republic Day on the 26th of January, Independence Day on the 15th of August, and Gandhi Jayanti on the 2nd of October. These are the only holidays the Union Government legislates as mandatory across every state and union territory.
Republic Day is celebrated in India on the 26th of January. It marks the day the Constitution of India took effect, in 1950.
Section 25 of the Negotiable Instruments Act, 1881, lets state and union territory governments in India declare additional local holidays on top of the national and central ones.
Sikhs mark Diwali as Bandi Chhor Divas, commemorating Guru Hargobind's release from imprisonment at Gwalior Fort by the Mughal emperor Jahangir and his arrival that day at the Golden Temple in Amritsar. Guru Nanak's Birthday separately honors the first Sikh guru, who was born in 1469.
Banks in India observe the 1st of April as a national bank holiday set aside for the annual closing of accounts, since it is the first day of the financial year.
Fixed central holidays in India, such as Christmas Day, Eid al-Fitr, and Diwali, are set on the same date every year for central government offices. Optional central holidays, such as Makar Sankranti and Karva Chauth, let individual offices choose a select number of days according to convenience.