This from the U.S. Embassy's www site. For Thai, I get 2-3 different lists off the www, so not posting.

Mac

http://bangkok.usembassy.gov/holidays.html

Official Holidays 2011

Month Date Day Occasion

December 26 Monday Substitute for Christmas Day

Official Holidays 2012

Month Date Day Occasion

January 2 Monday Substitute for New Year’s Day

January 3 Tuesday Special Holiday

January 16 Monday Martin Luther King, Jr.’s Birthday

February 20 Monday Presidents’ Day

April 6 Friday King Rama I Memorial and Chakri Day

April 13 Friday Songkran Day

April 16 Monday Substitute for Songkran Day

May 7 Monday Substitute for Coronation Day

May 28 Monday Memorial Day

June 4 Monday Visakha Bucha Day

July 4 Wednesday Independence Day

August 13 Monday Substitute for Her Majesty The Queen's Birthday

September 3 Monday Labor Day

October 8 Monday Columbus Day

October 23 Tuesday Chulalongkorn Day

November 12 Monday Substitute for Veterans Day

November 22 Thursday Thanksgiving Day

December 5 Wednesday His Majesty the King's Birthday

December 10 Monday Constitution Day

December 25 Tuesday Christmas Day

December 31 Monday New Year's Eve

For after hours emergency assistance, please contact the Embassy operator at 66-2-205-4000.

Don't forget, they are also closed the last Friday of every month.