The Four Seasons are known as Solar Festivals, the cross quarter days are marked by Fire Festivals, together they form The Wheel of the Year.
The Wheel Of The Year

Wheel of the Year

The Wheel of the Year

The Four Seasons are known as Solar Festivals, in that they mark a seasonal change caused by the Sun. The cross quarter days are marked by Fire Festivals and are usually celebrated as significant agricultural festivals. Together the Solar Festivals and the Fire Festivals make up the Wheel Of The Year. The Wheel Of The Year, is often broken into eight festivals, whether they are the eight Asatru Blots, Seasonal Festivals or Celtic Sabbats, and the observance of Solar energies at the solstices and equinoxes and the Fire energies on the cross quarter days, is a common theme throughout the world.

The Festivals of the Wheel Of The Year also represent the active and dormant states of nature, man and agriculture. Each of the festival days was ruled by a governing deity, whether a God or Goddess, with each region having its own associated deity. From planting to reaping to winter to summer... the seasons were of great importance to our ancestors, for their very existence depended upon good harvests, mild winters, enough rainfall.

Yule - Winter Solstice
Yule - Winter Solstice
Yule - Winter Solstice

The winter solstice, the rebirth of the Sun, is an important turning point, as it marks the shortest day, when the hours of daylight are at their least.
Ostara - Spring Equinox
Ostara - Spring Equinox
Ostara - Spring Equinox

It is traditionally the day of equilibrium, neither harsh winter or the merciless summer, and is a time of childish wonder.
Lithia - Summer Solstice
Lithia - Summer Solstice
Lithia - Summer Solstice

Midsummer or the Summer Solstice is the most powerful day of the year for the Sun God, this Sabbat glorifies the Sun God and the Sun, and fire plays a very prominent role in this festival.
Mabon - Autumn Equinox
Mabon - Autumn Equinox
Mabon - Autumn Equinox

Technically, an equinox is an astronomical point and, due to the fact that the earth wobbles on its axis slightly , the date may vary by a few days depending on the year.
Imbolc
Imbolc
Imbolc

This holiday is also known as Candlemas, or Brigid's Day. One of the 4 Celtic Fire Festivals. Commemorates the changing of the Goddess from the Crone to the Maiden.
Beltane
Beltane
Beltane

This festival is also known as Beltane, the Celtic May Day. It officially begins at moonrise on May Day Eve, and marks the beginning of the third quarter or second half of the ancient Celtic year.
Lammas
Lammas
Lammas

This is an Irish Gaelic name for the feast which commemorates the funeral games of Lugh, Celtic god of light, and son of the Sun.
Samhain
Samhain
Samhain

Samhain is one of our four Greater Sabbats, the highest holy day of witches. It is a cross quarter day, situated between Autumn Equinox and Winter Solstice.