Well, I'm a little late posting this, but here it is anyway.
Samhain is the Gaelic festival from which Halloween originated. The festival starts on November 1st, but festivities begin on the evening of October 31, because the sunset was dividing moment between one Celtic day and the next.
It's the end of the harvest season and the beginning of the darker winter season.
It's pronounced "SOW-win" (the first part rhymes with "how").
Here's a link to Muirgen's delightful explanation:
https://www.instagram.com/reel/CzEbBaWs5SL/?utm_source=ig_web_copy_link&igshid=MzRlODBiNWFlZA==