The new year began with the festival of Samhain. (Actually, it should be said here that the Celtic year was based on farming patterns and that the year began with the coming of winter.) At Samhain, the Celts believed, the veil between the worlds of living and dead was extremely thin, so thin that the spirits of the dead were able to come back across and walk with the living. It was the duty of the living to invite these spirits to join in the Samhain feast, so as not to feel chagrined or left out and, therefore, wanting to do harm.
As winter progressed, the spirits of the dead went back to their own world and the living went into a deep freeze. The farming cycle began anew with the end of winter, which was marked by the celebration of Imbolc. A rough translation of this word from the Celtic languages approximates the readying of ewes to give birth. The arrival of lambs means that winter is over and life can return, especially to the ground.
The third, and perhaps most famous, festival is Beltain, which takes place on the first day of summer. This festival honors partnerships and, especially, fertility. It took place when the first blossoms of May appeared and involved elaborate celebrations of the relationship between earth and life. Again with the focus on the land and animals, Celts would transfer their animals from winter to summer pastures, making sure to drive the animals between the fires of Beltain to keep evil spirits.
The last festival of the year is the longest one: Lugnasa. This monthlong celebration honors the bringing in of the harvest and reveres the gods for protecting the people and their crops and animals.
These ancient festivals have parallels in todays world. Ancient Festivals of the Celts
Samhain and its tradition of spirits walking the earth is alive and well in modern-day Halloween.
Imbolc and its emphasis on new life and winters end can be found to be similar to Candlemass, a Christian celebration on February 2 (not to mention the more secular Groundhog Day).
Beltains purpose has been co-opted by May Day, which is celebrated in many countries as the beginning of the end of spring and the forerunner of summer.
Lugnasa, as well, has parallels in todays holidays, especially the Jewish holiday of Shavuot (which is actually in the summer but celebrates the end of the harvest).