Samhain
Begins - Evening of October 31
Ends - November 1 or November 11
Celebrates - Traditional first day of winter in Ireland
Related to Hallowe'en, All Saints Day, All Souls Day
Yule
Significance - Marks the nativity of Jesus, the Ancient Midwinter, and the Astronomical beginning of Winter.
Date - December 25 or the Solstice of Winter
Celebrates - Burning Yule Logs, Feasting, Caroling, Being with Loved Ones.
Related to Nativity of Jesus, The Solstice, Quarter days, Wheel of the Year, Winter Festivals
Imbolc
Date - February 1/February 2
Related to Candlemas
Ostara
Significance - Marks the Celtic Mid-Spring, and the Astronomical beginning of Spring
Date - Vernal Equinox on March 20 or 21 in the Northern Hemisphere
September 21 or 22 in the Southern Hemisphere
Celebrates - Celebrating the new life and fertility of the land
Related to Easter, Eostre, Vernal Equinox, Quarter days
Beltane
Date - The beginning of May, 1st of May
Celebrates - Traditional first day of summer in Ireland, Scotland and Isle of Man
Related to Walpurgis Night, May Day
Midsummer
Significance - Marks the Ancient middle of Summer, Astronomical beginning of Summer, and the nativity of St. John the Baptist.
Date - June 21, 24, 25 or a date close to the Summer Solstice on June 20-23
Related to Summer Solstice, Quarter days, Nativity of St. John the Baptist
Lughnasadh
Date - 1 August
Celebrates - Traditional celebration of first fruits / first harvest
Related to Lammas
Mabon
Significance - Marks the Celtic Mid-fall, and the Astronomical beginning of Fall
Date - Autumnal Equinox on September 21 or 22 in the Northern Hemisphere
March 20 or 21 in the Southern Hemisphere
Celebrates - Thanksgiving for the Fruits of the Earth
Related to The Harvest festival, Equinox, Quarter days, Thanksgiving
Related Reading
Wheel of the Year - Wikipedia page about the different Pagan Sabbats.
The Witches' Sabbats