It is On October 31st. But it has nothing to do with Halloween, well it has a few things in common with Halloween, but Halloween is
an adapted Holiday, it was adapted from Samhain.
Samhain is the Fall equinox, and is celebrated as a Sabat by Wiccans and pagans. It is the end of the Harvest Season and the 'veil' between the world of the living and the 'world of the dead' is especially thin.
I made a post on a few small ways to celebrate Samhain
Actually the whole thing kind of evolved into a big history lesson on Samhain.