This Halloween has a special feature: a blue moon.

Halloween falls on Samhain Eve, the day before the Celtic holiday of Samhain. This already gives the holiday acute spiritual significance. Samhain marks the day the Celts believed summer changed into winter. They began their festivities at sunset the night before. 

But this Halloween is especially significant, given that it also coincides with a blue moon.

What is a blue moon, and why is its appearance on Halloween notable?