Equinoxes, solstices and cross-quarter days mark the seasons of the year.
The cross-quarter days are days from the ancient Celtic calendar. They mark the start or end of a season according to the Celtics. The equinoxes and solstices were the mid-point, moving toward the ending of a season.
I like to follow the solstices, equinoxes and cross-quarter days. I also like using the Algonquin names of the full moons.
If you want to learn more about Celtic Quarter Days there are many books and websites which have information. This page at Archeoastronomy.com has good information. http://www.archaeoastronomy.com/seasons.html
Imbolc – Cross-Quarter Day – Spring starts
Since the Celtic year was based on both lunar and solar cycles, it is most likely that the holiday would be celebrated on the full moon nearest the midpoint between the winter solstice and vernal equinox, or when the primroses, dandelions, or other spring flowers rise up through the snow.
Vernal equinox – Spring equinox – mid-point
Beltane – Cross-Quarter Day – Summer begins
Since the Celtic year was based on both lunar and solar cycles, it is possible that the holiday was celebrated on the full moon nearest the midpoint between the spring equinox and the summer solstice. The astronomical date for this midpoint is closer to May 5 or May 7, but this can vary from year to year.
Summer Solstice – Summer mid-point – days grow shorter after today
Lughnasadh – Cross-quarter Day – Autumn begins
Lughnasadh marked the beginning of the harvest season, the ripening of firstfruits, and was traditionally a time of community gatherings, market festivals, horse races and reunions with distant family and friends. Among the Irish, it was a favored time for handfastings – trial marriages that would generally last a year and a day, with the option of ending the contract before the new year, or later formalizing it as a more permanent marriage.
Autumnal Equinox – Autumn mid-point
Samhain – Cross QuarterDay – festival marking the end of the harvest – beginning of winter
Winter Solstice – Winter midpoint – days grow longer after today