The date of Diwali is based on the Hindu calendar, which has solar sidereal years and lunar months. Although the festival is worshipped on exactly the same set of days across India, it falls in different months depending on the version of the Hindu calendar being used in the given region.

The Amanta ("ending on the no-moon") version of the Hindu Calendar has been adopted as the Indian national calendar. According to this calendar, which is prevalent in southern India and Maharashtra, deepavali falls in the middle of the month of Ashwayuja. According to the Purnimanta ("ending on the full-moon") version prevalent in northern India, the 5-day celebration is spread over the last three days of the month of Ashwayuja and the first two days of the new month of Kartika. According to this calendar, the festival of Deepavali marks the new year's day of this calendar and is therefore an especially significant festival.

In the Gregorian calendar, it falls generally in the months of October or November. In 2005, the new moon day which is the third and most important day of the festival fell on November 1. In 2010, it will be celebrated on November 5th (Friday).