The Celts have always celebrated the complementary forces of existence such as darkness and light, summer and winter, night and day, death and life, and especially their transitions. They saw them as unities rather than opposites. I think this is very important to understand the Sacred Quadrilateral and the Troménie at Locronan.
For the celts, the day started at sundown and ended at the next sundown. They followed a lunisolar calendar, attempting to synchronize the solar year and the lunar month. The Hebrews, the Buddhists, the ancient Greeks, the Hindus and the Tibetans had a similar calendar. Such a calendar is far from accurate, and every third year, an extra month had to be intercalated to make it workable, among others.
Their year was divided in two parts of six months, winter and summer, dark and light. Winter from the first of November until the end of April, summer from the first of May until the end of October, subdivided in a kind of “end of winter” (what we would call spring) from the first of February until the end of April; and an “end of summer” (what we would call autumn) from the first of August until the end of October.
It seems that the “real“ solstices and equinoxes didn’t play a big role in their calendar, because they don’t fit in this system. Their solstices and equinoxes fell in the middle of the month (solstices on June 15 and December 15, equinoxes on March 15 and September 15).
They had four main “festivals: November 1 (water), February 1 (earth), May 1 (fire) and August 1 (air). Keep in mind that their November 1 started with the sunset of October 31, and so on.
This explanation is based on the Coligny Calendar, found in Coligny,
in 1897, one of the rare written Celtic calendars ever found. Ain, France