December to March is the best time to visit the Eastern Gulf: the seas are mostly calm and the skies mostly clear. Pattaya and Koh Samet are year-round destinations. Many places on Koh Chang and the other islands in its archipelago close down during the rainy season. The big car ferries, however, continue to run on a limited schedule, and the larger resorts and many hotels stay open, offering cheaper rates.
On the Western Gulf side the monsoon season runs from late October through December; prices can be halved (except between Christmas and New Year's). Peak season in the Western Gulf runs from January through early July. But even during the off-season flying to Koh Samui is still convenient, and the island is beautiful even with cloudiness and rain. Cha-Am and Hua Hin are busy year-round, particularly Hua Hin, a historical city that has more to offer than just the beach.
The peak season on the Andaman Coast is November through April. The monsoon season is May through October, during which high seas can make beaches unsafe for swimming, though hotel prices are considerably lower.