Four important dates occur each year, regardless of one's culture or the type of calender one prefers. These four dates neatly divide the year into four seasons. They are: vernal equinox, summer solstice, autumnal equinox, and winter solstice.
Vernal equinox marks spring; summer solstice, summer; autumnal equinox the fall; and winter solstice for winter.

These are not arbitrary dates, but are observable events in the cosmic clockwork of our planet's yearly orbit around the sun. Let's say that every day (night and day included) is 24 hours long.  Sunrise marks the beginning of day, sunset, the beginning of night. On the two equinox dates, the day and night are of equal length, 12 hours between sunrise and sunset; twelve hours between sunset and sunrise. Spring equinox is drawing near, with equal night and day. As the days move past the equinox toward summer, they will gradually become longer, the nights shorter, until the longest day of the year, the summer solstice. After the solstice, the days gradually become shorter until they are equal to the night again at the fall equinox. After the fall equinox, they continue to grow shorter until the shortest day of the year, the winter solstice. We light candles and put up tiny little lights to see us through the dark season. The days once again grow longer back to the spring equinox, when day and night, wherever you are on earth, are of equal duration.

In ancient times, architects and astrologers consulted, and oriented their sacred structures to mark where the sun rose and set on these special dates.