The Wheel of the Year
By watching the earth we know that time does not exist on a linear plane. A year does not start on January 1 and end on December 1. Time is circular, always coming back to the same spot eventually. A Pagan calendar has 4 or 8, depending on your tradition, holidays through out the year. Though they may be marked by dates on a calendar, they are marked much more deeply by the rhythm of the earth and the movement of the sun.
Winter Solstice
Imbolc
Ostra
Beltane
Summer Solstice
Lammas
Mabon
Samhain