Sabbats are the annual celebrations that Witches observe, marking the change of the seasons. There are a total of eight Sabbats spaced roughly equally around the year. There are three Fertility Celebrations, three Harvest Celebrations, and two Solstices. As the Wheel of the Year has no beginning nor ending, I have chosen to list them in order starting at the beginning of the secular calendar.
Unlike the holidays of other Faiths, the Sabbats are inextricably interconnected. The current Sabbat leads into the next, and is connected to the prior. Not only that, but each Sabbat opposes and is balanced by the Sabbat directly across from it on the wheel.
This can easily be seen with Yule, the Winter Solstace, and Litha, the Summer Solstace. Yule is the longest night of the year, while Litha is the longest day. Beltane and Samhain are similar, in that Samhain is a celebration of the season of Death, while Beltane is a celebration of Conception. Each extreme balances the other. Mabon and Ostara tend to be viewed more for the balance, since the hours of day and night are equal, but even there, we have the opposing forces of Spring and Fall.
Following is a list of Sabbats, and links to more information about each.
Imbolc is the beginning of Spring. It is celebrated on or around February 2nd. There are many tales surrounding Imbolc, and its Matron Goddess Brigid/Brigit/Breed. This is the first of the Fertility Celebrations. The Earth is coming back to life from its long Winter dormancy.
Ostara is the Vernal Equinox, celebrated around March 21st. While your calendar says that it is the first day of Spring, it is actually the middle of Spring.
Beltane is a Celtic Holiday, and as with all Celtic Holidays, it starts at dusk on one night, and goes until dusk of the next night. This is why there is so much confusion about the date. Beltane starts at dusk on April 30th, and goes to dusk May 1st. This is the first day of Summer!
Litha. Midsummer. The Summer Solstice. Again, your calendar says this is that this is the start of Summer, but as the Old Name implies, it is actually the middle of Summer. From here on out, the days start getting shorter. It is celebrated around June 21st.
Lughnasadh. Lamas. The Feast of Lugh. Celebrated on August 2nd, this is the first Harvest Celebration. The grains are being harvested, and that calls for a celebration!
Mabon is the Autumnal Equinox. Celebrated around September 21st. The vegetables in your garden are ripe, and ready for harvest.
Samhain. All Hallows Eve. Halloween. October 31st is the last of the Harvest Celebrations, and the beginning of Winter.
Yule. Jule. Midwinter. This is the Winter Solstice, and the longest night of the year. It is celebrated around December 21st, and was adopted by the Christian Church as the birthday of their Savior.