The cold season is generally from November to February. In Bangkok this means it might get as cold as 60F at the coldest point of the night, but more likely 65F. In the day it might average around, say, 75F. Sometimes maybe as low as 70F and higher up to 85F. In Bangkok you will only notice about 1 to 3 weeks of unusually cool weather, and then it’s back to the normal heat you’re used to. It only gets hotter the further south you go.
In the north of Thailand, say Chiang Mai and Chiang Rai, it might get cold enough that you’ll see frost. On a really windy cold night it might even get as low as 40F (or at least it will feel that cold). Although many of us foreigners vacation in Thailand to flee the winter in our home country, Thais like to travel to northern Thailand to experience what cold weather is like. Even frost on the grass is a white wonder winterland for them. I’ve never heard of snow or even hail in Thailand, but perhaps it happens on very rare occasions? The southern-most regions of Thailand generally don’t have a cold season.
It does on occasion rain during cold season, like 2-3 times per month.