I wikied it

Halloween has its origins in the ancient Celtic festival known as Samhain (Irish pronunciation: [ˈsˠaunʲ]; from the Old Irish samain).[1] The festival of Samhain is a celebration of the end of the harvest season in Gaelic culture, and is sometimes[2] regarded as the “Celtic New Year.”[3] Traditionally, the festival was a time used by the ancient Celtic pagans to take stock of supplies and slaughter livestock for winter stores. The ancient Gaels believed that on October 31, now known as Halloween, the boundary between the living and the deceased dissolved, and the dead become dangerous for the living by causing problems such as sickness or damaged crops. The festivals would frequently involve bonfires, into which bones of slaughtered livestock were thrown. Costumes and masks were also worn at the festivals in an attempt to mimic the evil spirits or placate them.

I don’t get it, what is it celebrating?