November to April is Kata's high season — sunny, dry, calm Andaman Sea, and the safest swimming. December and January are the busiest months and the most expensive; February and March are slightly quieter but the weather is still excellent.
May to October is the south-west monsoon. Heavy showers (usually short, often dramatic) and a building swell turn the beach into a surf spot rather than a swimming one. Hotel prices drop by 30–50% and the town is noticeably emptier. Red flags go up on dangerous days — heed them, the rip currents are serious.
The shoulder months of April–May and October–November can offer the best balance: lower prices, still mostly sunny, and small enough crowds to enjoy the beach.