as per the Hindu lunisolar calendar.

The festivities begin two days before amavasya, on Dhanteras, and extend two days after, until the second (or 17th) day of the month of Kartik. (According to Indologist Constance Jones, this night ends the lunar month of Ashwin and starts the month of Kartik - but see this note and Amanta and Purnima systems.) The darkest night is the apex of the celebration.