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 system 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 (Litha) 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.
The significance of having a circular calendar is huge. It reconnects us with the natural world and our own life cycles. We are part of the web of life and by connecting our holidays to our solar calendar we are re-connecting with this natural web. Each holiday has it's origin from the indigenous people of Europe. (the holidays and seasons in the southern hemisphere is reversed). They planted at Spring Equinox (Ostara) and celebrated growth. At mid-Spring they celebrated fertility at Beltane and at Summer Solstice the expansion of the light with the longest day of the year, Litha. The next two holidays, Lammas (mid summer) and Mabon (Fall Equinox) were the harvest festivals, then Samhain honored the death cycle and our deceased relatives. At Yule, the Winter Solstice, people of old celebrated the return of the sun finding gratitude for the light returning. Lastly they noticed the first awakening of spring and the end of the long winter at Imbolc (Aka Groundhogs day.
Many of the Pagan holidays were adapted by Christians during the conversions- Christmas (with the Tree, mistletoe, 5 pointed star and ornaments) is at Winter Solstice and Easter is based on Ostara (where bunnies and eggs originated). Halloween is a modern cultural appropriation of honoring the dead at Samhain, much like Dia de Los Muertes in Latin America.
When we realize that we are part of the cycles of Earth, returning to a circular calendar makes sense. We live on a round planet, that goes around a central sun, in a circular solar system, that is actually traveling at great speeds around the galaxy. We are constantly in circular motion. Why not accept that relationship with our holidays, too?