Wiccans, Druids and other modern pagans use the Wheel of the Year liturgical calendar. It is based on the agricultural calendar of Western Europe and defines a cycle, birth/sowing, growth, decline/ripening and death/harvesting and storage of crops. Some NeoPagan traditions attach cosmological storylines to these activities. The interpretation of the calendar varies from tradition to tradition, but most are of agrarian origin.
The Wheel of the Year is made up of 8 observances, known in the Wiccan community as Sabbats. Although their names and specifics vary by tradition, they are known by most as Yule or Midwinter, Imbolc, Ostara, Beltane, Midsummer or Litha, Lammas or Lughnasadh, Mabon, and Samhain.
There are eight observances, four quarter days or Lesser Sabbats are marked by the equinoxes and solstices. They occur on the 20th, 21st or 22nd of the month when the sun is at zero degrees in the sign associated with the Sabbat or the moment the sun enters the sign. That leaves the four Greater Sabbats, also known as the Cross Quarter Days. They are approximately halfway between the equinoxes and solstices when the sun is about 15 degrees in their respective signs. The modern fixed days of each of these is usually the first of the month and usually is a two-day festival beginning the evening before. Some people calculate these days astrologically so dates may vary. You can look into your own astrological calculations for more information.