A very interesting phenomenon related to this: there is a major holiday near *all* of the season changes (for the United States, anyway), and one near most of the mid-season markers as well. Something about celebrating change and all that, I suppose. A great astronomy professor I once had, Jim Kaler, clued me into this.

Autumnal Equinox: Labor Day (1st Monday in September)

Mid-Fall: Halloween (October 31st)

Winter Solstice: Thanksgiving (4th Thursday in November), Christmas (December 25th), Passover (late March, early April), Kwanzaa (26th-31th of December) and New Year's Day (January 1st)

Mid-Winter: Valentine's Day (February 14th), President's Day/Washington's Birthday (3rd Monday in February)

Vernal Equinox: St. Patrick's Day (March 17th), Easter (Early April)

Mid-Spring: Memorial Day (Last Monday in May), Mother's Day (2nd Sunday in May)

Summer Solstice: Independence Day (July 4th)

Mid-Summer: ????

Notice that most of the big holidays (including all 3 religious biggies) occur when it is coldest, and relatively few occur in the summertime...I suppose when it's warm, you don't need a special reason to break the monotony and celebrate.