Here's a link to holidays in Thailand for 2015. Just remember that when they say "observance" it isn't an actual holiday just a day they recognise

www.timeanddate.com/holidays/thailand/

Also, be aware that not all institutions will have all holidays, eg:

1) Financial institutions (FI) - if you're looking for when most banks are open - there's a list of FI holidays on BOT's website:

www.bot.or.th/English/FinancialInstitutions/FIholiday/Pages/2015.aspx

2) Government officials such as immigration may not also have all the holidays.

For example: Friday 1 May, immigration at Chaeng Wattana complex, Bangkok is NOT closed.

I checked with them in person today (and yesterday by phone), as my extension of stay based on spouse is due on 3 May (Sunday).

Cheers
Fletch