The Celtic year was divided into four main parts based on the farming cycle. The Celtic year begins with Samhain. In Ireland the year was divided into two periods of six months by the feasts of Beltane (May 1) and Samhain ( November 1), and each of these periods was equally divided by the feasts of Imbolc (February 1st or 2nd), and Lughnasadh (August 1)
Samhain (pronounced 'sow'inn' and the word for November in some Gaelic languages) which is a celebration of the end of the harvest season in Gaelic culture. It was also the time of year when the veils between this world and the Other world were believed to be at their thinnest: when the spirits of the dead could most readily mingle with the living once again. Later, when the festival was adopted by Christians, they celebrated it as All Hallows' Eve, followed by All Saints Day, though it still retained elements of remembering and honoring the dead (not forgetting the festival of Harvest Thanksgiving which tends to be a movable feast these days)
Imbolc most commonly is celebrated on February 2nd, since this is the cross-quarter day on the solar calendar, halfway between the Winter Solstice and the Spring Equinox in the northern hemisphere. Among agrarian peoples, Imbolc has been traditionally associated with the onset of lactation of ewes, soon to give birth to the spring lambs. The Christian Feast of the Presentation of Jesus at the Temple celebrating an early episode in the life of Jesus, and falls on or around February 2nd.
Beltane (the Gaelic names for either the month of May or the festival that takes place on the first day of May) is a festival celebrating the beginning of summer and open pasturing in Ireland and Scotland. There were similar festivals held at the same time in the other Celtic countries of Wales, Brittany and Cornwall. The festival persisted widely up until the 1950s, and in some places the celebration of Beltane continues today. Pilgrimages to holy wells are traditional at this time.
Lughnasadh marked the beginning of the harvest season, the ripening of first fruits, and was traditionally a time of community gatherings, market festivals, horse races and reunions with distant family and friends. On mainland Europe and in Ireland many people continue to celebrate the holiday with bonfires and dancing. The Christian church has established the ritual of blessing the fields on this day and in some English-speaking countries in the Northern Hemisphere, August 1st is Lammas Day (loaf-mass day), the festival of the first wheat harvest of the year. On this day it was customary to bring to church a loaf made from the new crop.