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