Diwali date is decided using the Hindu calendar Vikram Samwat. According to this, Diwali is celebrated during the new moon (darkest night) at the end of the Hindu lunar month of Ashvin and the start of the month of Kartika. read more
Hindu festivals fall on the same date every year according to the Hindu calender(which is a lunar one). The dates are decided according to the moon's appearance. For example, diwali is celebrated on Karthik amawasya (Karthik being one of the months and amawasya being new moon) every year. read more