Also called All Hallow's Eve, Ancestor Night.  It is the most important of the four "greater Sabbats".  The third and last of the three pagan autumn harvest festivals. Observance begins at sundown on October 31.

We are no more than magical shadow‑shapes that dance, limned by the Sun‑filled lantern held at midnight by the Mistress Moon...
LXVIII Rubaiyat
Shadow Shapes
O sister of sorrowful gaze! ...
You move in another dominion
And hang o'er the historied stone:
Unpruned in your beautiful pinion
Who wander and whisper alone.
Fairy Pendant
W. B. Yeats

Saying Good-bye