There's no snow in Thailand at the moment, and I'll bet my Macbook it won't happen, but according to my research It did snow once!
The only ever official (I use that word loosely) snow recorded in Thailand was in Chiang Rai on 7th January 1955 (see the picture below). According to the notice on this picture, the snow (sort of) came after rainfall at 6pm.
Basically, there was hail after the rain that produced a snow-like blanket on the ground, which remained for about 14 hours. Sadly, 18 hours later the snow had disappeared.
This is fiercely contested by snow geeks who say the “snow” was hail and therefore doesn't constitute a proper snowfall. However, residents beg to differ and proudly boast their claim to snow fame.
Either way this is as close as you're going to get, because this is the one and only time it has ever happened. It does look a lot like snow in the picture though, doesn't it?
Those who live in the North will know that it does get pretty chilly come December-February, and temperatures have been known to dip below zero during the night.
Bangkok doesn't get that cold by a long shot. It hit its coldest record in 30 years on January 23rd 2014, when the temperature fell to 15.6 Celsius – not really that cold compared with the UK, US and Europe in January, but quite a shock for the residents who are used to more like 30 Celsius year-round.
The lowest ever recorded temperature in Bangkok was 9.9 °C (49.8 °F) in January 1955, which coincidentally was the same year snow fell in Chiang Rai.