What is Christmas like in Thailand?
The short answer is that it is a non-event. Thailand is about 85% Buddhist, 14% Muslim, and the remaining 1% covers all other religions. So you can see that its not a festival celebrated here in Thailand, and Christmas Day is a normal working day. The main celebrations in December relate to HM the Kings Birthday on 5th December, and celebrations continue until the end of the month.
In Bangkok and other major cities and tourist areas, you will see some Christmas decorations and Christmas trees, mainly in shopping malls and hotels. This is done both as a commercial exercise and also part of the Thai spirit of fun. In December, do not think that all the decorations and lights you see are just for Christmas - most are in fact illuminations celebrating HM the King's birthday on 5 December, and the lights stay on until New Year. In Bangkok in particular, you will see elaborate decorations in many areas celebrating the King's birthday.
On Christmas Day, if its a weekday, businesses, government offices, attractions, and shops will all be operating normal business hours. Shops and stores will be busy, as they are every day in the run up to the New Year break.
Transport runs a normal service, but all forms of transport during late December are very busy as it is peak season, and Thai, as well as tourists, are also preparing to travel at this time
There are several churches around Bangkok (and in other cities) where you can take part in Christmas religious celebrations, but most services will be in Thai.