Four great feasts divided the Celtic year into four parts:

Imbolc
Celebrated on February 1. It was associated with the goddess Brigit, a Mother-goddess and protectress of women in childbirth.

Beltane
Celebrated on May 1. Beltane (also spelled Beltine, Beltain, Beal-tine, Beltan, Bel-tien, and Beltein) means "bright fire" and in the past two fires were lit by Druids in honour of Bel. Later it came to be associated with May Day (Calan Mai).

Lughnasadh
Also known as "Lammas", 'the feast of the god Lugh', was celebrated on August 1 in connection, it would appear, with the harvest.

Samhain
The most important of the four feasts, was celebrated on November 1. It is now associated with 'All Souls' Day' (All Hallows' Day), which today is mostly celebrated the night before, on Halloween (All Hallows' Eve). This would not be inappropriate because the Celts, and later relgious groups, including the Christians, reckoned a daily unit as running from sunset to sunset, so that the night before was counted with the date of the following day.