Checkout one of the weather sites (yahoo etc.), for an almanac of annual temperatures and rain fall at your chosen location. Bear in mind that there can be a significant difference between the southern most tip and most northerly Thailand.
The rainy season runs from April – October, but you might like rain.
You should be fine for Feb. Last time I was in BKK was Feb and it was sunny every day. In fact I can`t remember many times when I`ve been there and it has been tipping with rain apart from when they`re having a thunderstorm. I`ve been in Ko Samui in Feb and that was fine too.
Been to Thailand about 4 times now. Generally Bangkok and southern areas including Koh Panyang, Koh Samui and Koh Chang. We go about Jan/Feb and found the weather nice and sunny and not too hot about 24-25C tops.
Bangkok is a bit humid but everywhere else is bearable,
Over 4 years only got caught in two thunderstorms but there very shortlived and over quickly.
The rainy season varies to where your going in Thailand but as Hymie says it generally April to October and occasionally wanders in to November.
I've just come back from Thailand, supposedly the middle of their rainy season, we barely saw any rain, one storm which was torrential, lasted about 30 mins and was then dry as a bone an hour later. We stayed in Hua Hin, the rain seems to bypass the area somehow. It's a wonderful country, you'll enjoy it whenever you go.