Besides Christmas and New Year's eve that is.
5 December (a Sunday) is HM the King's birthday. Banks and government offices wil be closed the following day. HM has said he doesn't mind if people drink on his birthday, but you never know. The government may force the bars to go dry, which means they will probably close. (Hotel bars are usually exempt.) If the bars do close, the freelancers will still be out.
10 December is Constitution Day, also with the banks and government offices closed. Bars should be open.
25 December of course is Christmas, but is an unofficial holiday in the Buddhist nation. Bar will be packed - and expect the bar fines to be increased (maybe doubled).
31 December has been the official New Year since 1941. Thais celebrate it as in other countries, but again expect bar fines to be greatly increased. The bar owners won't ever let a good opportunity pass by.
http://www.thaiworldview.com/feast/feast.htm