Believing that the starting point of all festivals is Sanskrit puranas written by Brahmins because Brahmins say so, is like believing Narendra Modi fought and brought a crocodile home because he has written so in his Bal Narendra comic strip. Be a bit more critical, people.
Majority of Bahujans don't give a damn about puranic myths btw. They are gloriously oblivious of them.
Another point. Puranas started to be written around fourth to fifth century and really picked up in later centuries. Do not tell me people didn't celebrate festivals in ancient times.
Do ask people why they celebrate a festival before imposing your own meaning on them.