The earliest known literary reference for Diwali is found in Neelmat Purana, composed in Kashmir around 5th to 8th Century AD.

Neelmat Purana composed in Kashmir
describes in detail the “Deepmala”
festival with the following features : –
All-round illuminations, hoisting up of festoons, Feasting with Brahmins, relatives and dependents, Gambling, Music, Passing the night in the company of ladies,Wearing of rich apparel and jewels, Presenting new garments to friends, relatives, Brahmans and the servants.

It also mentions the day of occasion as the new moon day of the Kartika month of the Hindu Lunar Calendar.

This also point to the fact that the myth of Rama associated with Diwali is a later construction and it’s possible that it is a celebratory eulogy of Pushyamitra Sunga upholding the Brahmanical order in society.

Diwali has been a prominently brahmanic festival, and like most of the brahmanic practises of the time, it was never a part of the Buddhist faith.

However, among various castes there were similar observations of some rituals on the 5 days of diwali in Ashwin and Kartik month of “Hindu” Lunar calendar.

This might point to the fact that the days chosen for Diwali by the Brahmans were somewhat taken from the local cultures already having feasts or totem worships.

But at the same time we can not rule out that the local cultures were in a way “sanskritising” themselves in order to claim legitimacy in the brahmanic order because there are references to Kolis, Bhils and other lower castes eating meat and lighting earthen lamps in corpses yards during this 5 day week long festival.

As far as it is known to me ( anyone can point out if I am wrong), mostly Buddhism at that time has remained aloof and maintained distance from local cultural and ritual practices, which had elements of Brahmanic order in them. So it is highly likely that there was no diwali in Buddhism.

There are clear references to Jains celebrating Diwali as the day of Mahavira attaining moksha/nirvana and the festival of Dhanteras as a prominently mercantile community festival, as Jainism at that time was a prominent religion among the mercantile communities of that time.

As far as lighting lamps is concerned, it is to be understood that in those times it was the only plausible and obvious way of celebrating grand occasions, probably the reason that many castes were observing the same ritual of lighting lamps but that doesn’t necessarily points to the fact that they were celebrating Diwali, as per the Brahmanical mores.