An interesting question. There has never been any recorded snowfall in bangkok. So it must have been before man began keeping records. Recent ice ages such as the one 19000 years ago would still not have seen ice or snow in low lying areas near the equator so i suspect you would have to go back to when the tectonic plates were positioned differently and asia was linked with america, millions of years ago.
We got a dusting after midnight on Wednesday. Not sure if it met the .1in requirement to be considered measurable snow. Otherwise it would be last January. Each of the last two winters have been below average though. Average is around 9-10in per winter.
DC, we got a dusting yesterday. First snow this winter which is weird and I'm not complaining.... Not that we get a lot, but usually a few multi-inch snowfalls per year.