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.