What old Celtic celebration coincides with Halloween?

It is widely believed that Celts celebrated the first Halloween.

Halloween's origins date back to the ancient Celtic festival of Samhain (pronounced sow-in).

The Celtic new year was celebrated on November 1 and they believed the night before, October 31, the ghosts of the dead returned to earth. On the night of October 31, they celebrated Samhain, when the ghosts returned, by building bonfires and wearing costumes.