Updated: Jun 15, 2020
There are 8 sabbats, situated around the solstices and equinoxes. Here is a quick explanation of each one and the date it falls on or near. It does slightly change from year to year if you base your celebrations on the actual events, rather than set dates. To keep things simple and easy, you can always celebrate on a set day, which is what I've listed as the dates here. A quick internet search will tell you the astronomical date of each sabbat every year.
Yule is the winter solstice, the shortest day and the longest night of the year, and the official start of winter. After this night the days will start to get longer again. This is the holiday that Christmas was intended to replace. Although the world is already cold and winter's grip is starting to tighten, the coldest part of the year hasn't arrived yet and up to now, it was simply the coldest parts of autumn.
Imbolc is the halfway point between the winter solstice and the spring equinox. Winter is halfway over and in some areas, you may already be seeing bits of spring showing through. Groundhogs day falls on the same day, and his prediction is derived from Imbolc's ideals. The coldest days of the season usually fall in the weeks leading up to this, and sometimes a few weeks afterward.
Ostara is the spring equinox and the halfway point between the winter and summer solstices. Spring officially starts on this day and it's where Easter concepts sprung from. The earth is being renewed and reborn, spring animals are emerging and giving birth to the next generation.
Beltane, also called Mayday, is the halfway point between the spring equinox and summer solstice. Spring is in full roaring bloom in most places by this point. The first spring crops may even be maturing, and the earliest blooming flowers are in all their glorious colors. It's a very exuberant time of the year!
Litha is the summer solstice. It's the longest day and shortest night of the year and signifies the start of summer. Bonfires are the centerpiece of this Sabbat. The nights will start to get longer after Litha and the most important crops to ancient peoples were starting to ripen for harvest.
Lammas is the halfway point between the summer solstice and the fall equinox. It is first and foremost a harvest holiday. Some of the most important crops were ripening and hard-working people are putting in all their time to get the harvests pulled in and processed to keep them fed during the hardest months of the year.
Mabon is the fall equinox and is the pagan thanksgiving. The biggest harvests are done and put up for the winter, and it's time to celebrate the great abundance of food. It's a time to thank the gods and the earth for nourishing you. A large feast is a great way to celebrate this day, as well as preserving and putting away as much as possible for winter.
Samhain, probably the most well known of the pagan holidays, is the halfway point between the fall equinox and winter solstice. The harvests are finished and much of the plant life is dying or going dormant. Fall is well established and the fertility of the earth is going dormant. It's the time to harvest many of the animals on the farms to preserve for eating over the coming winter. For all these reasons, death was foremost in people's minds. The veil between this world and the next is thinnest and it's a time to commune with ancestors and loved ones.