I understand a lot of ancient people celebrated on the summer or winter solstices, being the longest and shortest days of the year. But for the Gaelic people of Ireland and Scotland, the two most important festivals was Samhain and Beltane, celebrated today on October 31 and May 1. But neither of those days are on a Solstices or Equinoxes, so how did the ancient people know which day to celebrate?

Did they just choose randomly depending on the weather? To my understanding, Samhain was the beginning of Winter for the Celts. But it can sometimes feel like winter as early as September. How did they know to wait until late October, rather than late September. I assume they didn't have our current concept of a 30 day, 12 month calendar.