January and February are mid-way through the cooler season and are often considered the best times to go to Thailand, especially after the Christmas vacations.
Things start to heat up over March, April and May so best advice is to head south to the beaches or search for less humid conditions amongst the higher hillsides of the north. Songkran, the Thai New Year festival, takes place in mid-April and although good fun, can lead to travel disruption in rural towns as everything shuts down for almost a week so people can chuck water at each other.
As heat turns to wet weather during May, June, July and August, you'll find it rains almost every day but only for about an hour or so which can sometimes be a welcome respite during the late afternoon or early evening.
September and October tend to find the worst of the weather and daily deluges are often strong enough to disrupt travel plans, particularly when trekking in more remote areas.
The end of the year, November and December, finds temperatures starting to dip and is one of the best times to go trekking in the north of Thailand although it's still advisable to avoid walking too long around the middle of the day.