Before we had a linear 12 month calendar people synced up to the cycles of nature: to both the solar and the lunar cycles. Most of us are familiar with a new moon or full moon. Every 29 ish days or so the moon travels around the Earth going from new to full (waxing cycle) or full to new (waning cycle) depending on where on the planet you live. Women's menstrual cycles were intrinsically tied to the lunar cycle, so much so, some women rather than say they are on their periods say they're on their moon. This is a reclaiming of a womans body with nature as her guide.
The solar cycle goes for an entire year as the Earth makes it's way around the sun. Just as the moon was essential for planting cycles, tides and rituals for growth or release, the sun's cycles guided our ancestors through the course of what was called The Wheel of the Year. The calendar was seen as circular like the solar systems orbits, rather than a straight line the way our Gregorian calendars are situated.
The wheel is sectioned into 8 pieces like a pie. The 8 holidays, called Sabbats are equally spaced.
On the X is Yule (Winter Solstice) and opposite that is summer solstice with the two equinoxes fall (Mabon) and spring (Ostara) on the other leg of the X.
The cross quarter holidays which fall mid season are Samhain (pronounced sow-en) AKA Halloween mid fall, opposite Beltane on May 1st mid spring, then there is the mid winter lesser know holiday called Imbolc (or Candlemas) Feb 1st opposite Lammas celebrating mid-summer on August 1st.