There are three long weekend holidays in Thailand during May 2013, however only two are for the general public. The next long weekends in Thailand: Coronation Day 4-6 May, Royal Ploughing Ceremony 11-13 May (*) and Visakha Bucha 24-26 May 2013.
(*) The Royal Ploughing Ceremony is only a holiday for government workers. However, you will find government offices including Immigration closed for the long weekend. State enterprises like the post office will also close