The Celtic wheel of the year was separated into eight segments, each corresponding with a celebration/festival. Because they count a day from one sunset to the next, a celebration would last from one night to the next.
The first four I listed are known as "fire festivals" due to the fact that a bonfire is one of the key parts of their celebration. These are always celebrated on the same day each year.
Samhain : Celebrated from October 31st-Novemeber 1st. This was once known as the Celtic new year. It marks the end of fall and coming of winter. At this time the veil between our world and the spirit realm was said to be the thinnest, and one could see spirits (faeries). It was also known as the day of the dead and people would "run wild".
Imbolc : Celebrated February 1st-2nd. This festival marks the beginning of spring and new life. It is associated with the goddess Brigid, and marks the end of winter.
Belantine : Celebrated April 30th-May 1st. They believed their god Bel (Cernunnos, the horned god) to die on this day but be reborn as the goddesses son. Traditionally people would weave red (for the god) and white (for goddess) streamers around the may pole as a way of symbolizing their ties and the cycle of life and death. This celebration is about fertility and the never ending cycle of life.
Lughnasadh : Celebrated August 31st- September 1st. This is known as a feast of the god Lugh, and celebration of the first harvest. It is said that the corn King (Lugh) dies at this time and is reborn in the spring to ensure a plentiful harvest for the next year.
The next four are festivals of light (Alban). These are based on the solstices. The Celts believed the equinoxes to be strong in magic. They were times for a balance between light and dark, but also balance between worlds.
Alban Arthuan : ( Light of Arthur) Is the winter solstice, and celebrates the return of the sun after the longest day of the year. It is celebrated similar to yule and Christmas.
Alban Eiler: (Light of the Earth) This is the spring/vernal equinox. This signified the start of spring, planting and fertility.
Alban Heruin : (Light of the Shore) This is celebrated during the summer equinox, on midsummer day, with games, picnics and light spirited fun.
Alban Elued : (Light of the Water) Is celebrated during the autumnal equinox.
I did not find much on how they celebrated the Festivals of light, besides the fact that they were times in which they performed magic rituals and spirit work. They were believed to be times in which one would see the faeries/spirits. I did not include dates for them, because they changed each year depending on the lunar calendar.
Celtic Wheel of the Year, By Jeri Ballast (cross referenced by my great aunt Carol S.)