Deciding the optimum time for rituals of Winter Solstice ( Yule ), Spring Equinox ( Ostara ), Summer Solstice ( Midsummer or Litha ) and Autumn Equinox ( Mabon ) is an easy prospect, because virtually all calendars, as well as the radio and TV announcers, will tell you exactly when these common seasonal beginnings occur. These seasonal holidays begin as the Sun enters the zero degree of each of the cardinal signs of the tropical (western) zodiac: 0 Capricorn for Yule, 0 Aries for Spring Equinox, 0 Cancer for Summer Solstice, 0 Libra for Autumn Equinox.
For the Greater Sabbats, however, the decision is not so clear-cut. If we make an even-spoked Wheel of the Year, these four Sabbats are at the exact midpoints of the equinoxes and solstices. They have been called "cross-quarters." According to the late and great astrological philosopher Dane Rudhyar, the cross-quarters are points of greatest momentum and release of power, which is in keeping with the Wiccan tradition of calling them "Greater Sabbats," as differentiated from the "Lesser Sabbats" that are more commonly known to the mundane public as the beginnings of our seasons.
It is interesting that if one truly watches the turn of the seasons, one realizes that it is the cross-quarters that are the true onset (beginning) of the seasons. At Imbolc or Candlemas, the lengthening of the days becomes evident and the first signs of spring appear-increasingly changeable weather lending some spring-like days, the tiny sprouts of crocus through the snow. By Ostara, spring is in full swing. At Beltane, we feel the onset of summer-flowers are blooming everywhere. The so-called beginning of Summer, is called MIDsummer in folk tradition! In the heat of August, at Lammas, we begin to see signs of approaching Autumn, while at Mabon the signs of fall are everywhere around us, and the air has turned brisk and cool. At Samhain, the leaves are gone, the earth turns brown and the onset of winter is clearly evident, while at Yule, though the TV announcer might say "first day of winter," we in the north are likely to be looking out at a world of white snow.
Unfortunately, over the passage of time, the celebration of the cross-quarter holidays, along with much of the folk tradition that accompanied them, faded from mainstream society. The clear cut astronomical markers of the equinoxes and solstices changed with calendar changes, while the cross-quarters were forgotten. For this reason, many of those of the Craft, who still celebrate the Greater Sabbats, continue to use the old traditional dates from long ago, even though from an astronomical/astrological standpoint, they are no longer accurate. This is not necessarily wrong-any tradition that is constantly repeated over time gains power, regardless of its astrological or astronomical correspondence. But, for those who are concerned about the maximum points of energy in the dance between Earth and Sun, "the points of greatest momentum and release of power" are the true cross-quarters. These are found when the Sun enters the 15th degree of each of the fixed signs of the western zodiac: Aquarius for Imbolc, Taurus for Beltane, Leo for Lammas, Scorpio for Samhain. In present time, these days usually occur a few days after the old traditional dates for celebrating the Greater Sabbats.
Following, then, is a list of the usual dates for the Sabbats, given both in ancient tradition, and according to zodiacal markers. Understand that even the equinoxes and solstices may vary by a day from one year to the next, because they are determined not by calendar date, but by Sun at the 0 degree of a cardinal point. Likewise, the Sun at 15 degrees of fixed signs may give or take a day on successive years.