Dates
Because it takes a long time to get to Bangkok from the states, we tend to go for around three weeks, so we'll be in town well before the wedding and for a week or two after.
With the wedding being on the 28th, it appears that the best time to fly to Bangkok would be the 25th or earlier. You can leave on the 26th, but unless you fly direct, there's a chance you might land on the 28th.
Unfortunately, December is the high season so it's important to book sooner rather than later. Also, we've noticed that ticket prices jump if you depart after December 14th. So if you have the time, leaving sooner is cheaper.