Keep in mind that tourists visit Thailand every month of the year! You do get more tourists during the late November - early February time period when the weather is better. This is also when the prices of hotels cost more, especially near Christmas and New Years holidays. Also, many flights from overseas cost more during this time period.
Even though other times of the year are either hotter or have more rain the weather is tolerable. Most of the time during the rainy season the rain stops in a couple hours. Sun comes out again, etc. During the hot season you still have tourists show up. Just drink more fluids, and walk on shaded side of the street. Best thing during this rainy - hot season is that hotel prices are much cheaper. Often you can get a room with a walk-in.
http://www.guidetothailand.com/thailand-travel-information/weather.phpVisit when you can get the time off. If funds are tight maybe save money on hotels and plane tickets by showing up March to June or August to October. From some countries, like Canada and the US, when the kids get out of school in June until they go back to school late August the airlines raise their prices. Before June and near the end of August the prices are a little lower.
Depending on the passport you use, you may be able to enter Thailand on a free visa exemption good for 30 days. Don't skip Bangkok! Plan on at least a few days in Bangkok at the beginning of your holiday and a final night or two at the end of your holiday. This makes your connections a little easier to make. Bangkok is cheaper than Phuket for hotels and local transportation. If you schedule a flight immediately out of Bangkok after arrival make sure you have enough time between connections. Keep in mind that some flights leave from a second airport in Bangkok now, Don Muang. This second airport is not close to Suvarnabhumi.
http://thaiembassy.ca/en/visiting-thailand/visas/tourist-visa-exemption
