Calendar of the Moon
Rituals for Beth / Poseideion II

The first month of the lunar year in the Celtic calendar is Beth, the month of the Birch. Roughly three out of five years, Beth is shortened because it shares a lunar cycle with Ruis. It coincides with Poseideion II on the Greek calendar. Their lunar calendar had twelve months, with Poseideion repeated when a "long" year was needed.

[Pagan Book of Hours]