Note on Start of Seasons

Only in the USA are the seasons said to begin on the respective solstice or equinox; that is, spring is said to begin on March 20, 2008. Elsewhere it is usually marked at the beginning of the month during which the solstice or equinox occurs, which would be March for spring in the northern hemisphere, or September in the southern hemisphere. But some countries mark it even earlier; see this link:
The Straight Dope: Is it true summer in Ireland starts May 1?

Note on Jewish Holidays

Jewish holidays begin the preceding day at sundown. Some Jews tack an extra day onto Passover and Rosh Hashanah. For more information on Jewish holidays, try this link:
Judaism 101: Jewish Holidays

Note on Sinterklaas, or St. Nicholas Day

Some people here in the USA celebrate St. Nicholas Day with their children, but there is often confusion as to which day it should be observed. Sinterklaas, or St. Nicholas Day, is observed in the Netherlands on December 5, and presents are opened in the evening. In Belgium, Sinterklaas makes his deliveries overnight on the 5th, and children wake up to find their presents on the morning of the 6th.

Date Holiday or observance Where it is observed
January 1New Year’s DayWorldwide
January 6EpiphanyWorldwide
Carnival beginsWorldwide
January 21Martin Luther King Jr. DayUSA
February 2Groundhog DayUSA and Canada
February 5Mardi Gras, or Fat TuesdayWorldwide
February 6Ash WednesdayWorldwide
February 7Chinese New Year (Year of the Rat)Worldwide (Chinese)
February 12Lincoln’s BirthdayUSA
February 14St. Valentine’s DayWorldwide
February 18Presidents DayUSA
February 22Washington’s BirthdayUSA
March 9Daylight Saving Time beginsUSA and Canada
March 16Palm SundayWorldwide
March 17St. Patrick’s DayWorldwide
March 20March Equinox (5:48 UTC)
Spring in the northern hemisphere
Fall in the southern hemisphere		Worldwide
March 21Good FridayWorldwide
March 23Easter SundayWorldwide
March 24Easter Mondayobserved worldwide but not widely in USA
April 1April Fool’s DayWorldwide
April 20Passover (7 days; begins the 19th at sundown)Worldwide (Jewish)
April 22Earth DayWorldwide
April 23Administrative Professionals DayUSA
May 1Ascension DayWorldwide
May DayWorldwide (pagan holiday is fading in popularity; some countries observe it like USA Labor Day)
May 5Cinco de MayoMexico
May 11Mother’s Day USA and others
Pentecost SundayWorldwide
May 17Armed Forces DayUSA
May 19Victoria DayCanada
May 26Memorial DayUSA
June 14National Flag DayUSA
June 15Father’s DayUSA and others
June 20Northern Solstice (23:59 UTC)
Summer in the northern hemisphere
Winter in the southern hemisphere		Worldwide
July 1Canada DayCanada
July 4Independence DayUSA
August 4Civic Holiday
A holiday with history (Toronto.com)		Canada
September 1Labor DayUSA
Labour DayCanada
September 7National Grandparents Day
Official National Grandparents Day site		USA
September 11Patriot DayUSA
September 22September Equinox (15:44 UTC)
Fall in the northern hemisphere
Spring in the southern hemisphere		Worldwide
September 30Rosh Hashanah, or Jewish New YearWorldwide (Jewish)
October 9Yom Kippur, or Day of AtonementWorldwide (Jewish)
October 13Columbus DayUSA
Thanksgiving DayCanada
October 16National Boss DayUSA and others
October 18Sweetest DayUSA
October 24United Nations DayWorldwide
October 31HalloweenWorldwide
November 1All Saints’ DayWorldwide
Dia de los Muertos, or Day of the Dead (2 days)Mexico
November 2Daylight Saving Time endsUSA and Canada
All Souls’ DayWorldwide
November 4Election Day
Election Day on Tuesdays? from infoplease.com		USA
November 11Veterans DayUSA
Remembrance DayCanada and others
November 27Thanksgiving DayUSA
November 30Advent SundayWorldwide
December 5Sinterklaas, or St. Nicholas DayNetherlands
December 6Sinterklaas, or St. Nicholas DayBelgium
December 7Pearl Harbor Day, 1941USA
December 21Southern Solstice (12:04 UTC)
Winter in the northern hemisphere
Summer in the southern hemisphere		Worldwide
December 22Hanukkah (8 days; begins the 21st at sundown)Worldwide (Jewish)
December 24Christmas EveWorldwide
December 25Christmas DayWorldwide
December 26Kwanzaa (7 days)
The Official Kwanzaa Web Site		 African American and Pan-African; USA and others
Boxing DayUK, Canada, New Zealand, Australia and others
December 31New Year’s EveWorldwide