Religious or not Diwali is the celebration of all that is good being victorious over evil.
Diwali is commonly called the festival of lights. If there is one occasion which is joy and jubilation for one and all, it is Diwali.
On the day of Diwali, Lord Rama returned to the capital of his Kingdom Ayodhya, after an exile of 14 years. He returned in triumph and glory after eliminating the Ravana hence the crackers and lights signify His victorious return and the triumph of good over evil. The people of Ayodhya were overwhelmed with joy, welcoming Rama through jubilation and illumination of the entire capital with lamps. Hence the celebration is affectionately known as the "festival of lights"
Along with Hindus the day is also celebrated by Sikhs, Jains and Buddhists for various other reasons.https://www.youtube.com/watch?v=HrrW3rO51ak&t=6s
Happy Diwali to all - light up the darkness!