Well, you've come to the right place.

For my own amusement, I've put together a list from different sources of the most common national and religious holidays observed in the U.S., along with some older holidays tied rather directly to points in the Sun's annual journey around the sky, and the astronomical dates which underlie many of them.

You've probably heard of the Spring and Fall Equinoxes (times of equal-length day and night) and the Summer and Winter Solstices (longest and shortest day, respectively) before. In current usage these each define the official beginning of a season -- for example, summer ``begins'' around June 21st. However a less-used parallel system holds that June 21st is actually Midsummer's Day, which then requires the start of summer to be in early May. This date and three others like it are known as the Cross-Quarter Days, because they are evenly spaced between the fundamental Quarter Days of the Solstices and Equinoxes. The Cross-Quarter Days thus mark the middle of each season under our current system, or seasonal boundaries under the alternative system. Due to the insertion of a Leap Day on February 29th every four years, the exact dates of these eight astronomical events shift back and forth, with a total range of about 54 hours.

Ancient peoples were very attentive to seasons and the Sun's position in the sky, because their livelihood depended on planting and harvesting at the proper times. All eight of the above-listed Days were observed as pagan holidays of one sort or another; a few, like Halloween, have survived to modern times in (somewhat) recognizable form. What is more interesting is the number of supposedly modern holidays which lie in close proximity to the same dates. Christmas (Winter Solstice) and Easter (Spring Equinox) are two obvious examples; one may make the argument that these holiday times were inherited. Others are quite surprising -- like Father's Day (Summer Solstice)! What are the folks at Hallmark hiding from us? Of course, not everything lines up, and the nearness of Election Day to All Saints' must be pure coincidence. Right?

  January 1          - New Year's Day
  January 5          - Twelfth Night
  January 6          - Eastern Orthodox Christmas
  	             - Twelfth Day 
  	                 or Epiphany
                         or Old Christmas
  January 20  	     - Inauguration Day
                     - Saint Agnes' Eve - woman dreams of future husband
  2nd New Moon after
     Winter Solstice - Chinese New Year (lunar calendar)

February 2-6	   1st Cross-Quarter Day

  February 2         - Groundhog Day
		     - Candlemas
  	                 or feast of the Purification of the Virgin Mary
  		               or of the Presentation of the Child Jesus
  	             - 40th Day of Christmas
		     - Imbolg / Imbolc (other Celtic names)
			 or Brighid
			 or Oimelc
  February 14        - Valentine's Day
  March 2nd Sunday   - Begin U.S. Daylight Savings Time (clocks 1 hour forward)
  March 15           - Ides of March - 1st month middle, Roman calendar
                         also assassination of Julius Caesar
  March 17           - St. Patrick's Day - sowing of peas in Ireland

March 19-21	   1st Quarter Day - Spring (Vernal) Equinox

  March 21	     - Ostara / Eostre (Saxon goddess of Spring)
  Full Moon after SE - Passover
  Following Sunday   - Easter
  Previous Friday    - Good Friday
  April 1            - All Fool's Day (old new year's day)

  April 30           - May Eve 
  	                 or Walpurgisnacht (witches' Sabbath)
                         or Walpurgis Night (after St. Walpurga)
  May 1              - May Day - May Poles, May Queens, May-dew, etc.
                     - Beltane / Bealtaine - Celtic bonfire festival

May 4-7		   2nd Cross-Quarter Day

  May 5		     - Cinco de Mayo (Mexico)
  May 2nd Sunday     - Mother's Day
  May last Monday    - Memorial Day ("Decoration Day", officially May 30)
  June 14            - Flag Day
  June 3rd Sunday    - Father's Day

June 20-22	   2nd Quarter Day - Summer Solstice

  June 21	     - Litha (Norse/Anglo-Saxon for "longest day")
  June 23            - St. John's Eve - European Midsummer celebration
  July 1             - Dominion Day (Canada)
  July 4             - Independence Day (U.S.)
  August 1           - Lammas ("loaf mass") - harvest festival
  	                 or festival of St. Peter's Chains
  		                  or of the Maccabees
  		                  or of the Gule ("mouth") of August
  	             - Lugnasad - Gaelic summer "games of Lug" (sun-god)
			 or Lughnasada
			 or Lunasa

August 5-8 	   3rd Cross-Quarter Day

  September 1st Mon  - Labor Day

September 21-24	   3rd Quarter Day - Fall (Autumnal) Equinox

  September 21	     - Mabon (Welsh for "son")
  New Moon after FE  - Rosh ha-Shanah (Jewish New Year)
  10 days later      - Yom Kippur ("Day of Atonement")
  October 2nd Monday - Thanksgiving (Canada)

  October 31         - Hallowe'en
  	                 or All Hallows E'en
  	                 or Hallowmas Eve
  November 1         - Dia de los Muertos
                         or Day of the Dead (Mexico)
                     - All Saints' Day
  	                 or Hallowmas or Allhallowmas or Allhallows
  	             - Samhain - Celtic feast of departing Sun & new year
  	                 or All Souls' Night
  November 2         - All Soul's Day - prayer for souls in purgatory
  November 1st Sun.  - End U.S. Daylight Savings Time (clocks 1 hour back)
  November 1st Tues
       after 1st Mon - U.S. Election Day

November 5-8	   4th Cross-Quarter Day

  November 5         - Guy Fawkes Day
  November 11        - Veterans' Day (World War I Armistice Day)
                     - Martinmas (death of St. Martin)
	  	         or Martinmas-in-Winter
  November 4th Thurs - Thanksgiving (U.S.)
  December 13        - St. Lucy's Day ("the year's midnight")
  December 19        - Saturnalia - Roman midwinter festival, 7 days long

December 20-23	   4th Quarter Day - Winter Solstice

  December 21        - Yule (Norse for "wheel") - Germanic 12-day feast
  December 24        - Christmas Eve
  December 25        - Christmas Day
  December 31        - New Year's Eve

Notes

1. Exact determination of dates for Chinese New Year, Easter, and Rosh Hashanah is a bit more complex than given here; see Explanatory Supplement reference below for details.

2. Pagan Quarter and Cross-Quarter Days (Wiccan Sabbats) may be in slight error; some practitioners observe them at the astronomically correct time, while others adhere to rigid dates (e.g., Feb 1 or Mar 21).

References

• The Astronomical Companion (1979) by Guy Ottewell, Universal Workshop
• ``Calendars'' chapter in the Explanatory Supplement to the Astronomical Almanac, 2nd Edition (1992), by L. E. Doggett
• American Heritage Dictionary (1965)
• The Alt.Pagan FAQ

Other Information

• Equinox and solstice dates and other cool stuff from the U.S. Naval Observatory
• Interfaith Calendar
• Holidays on the Net