Phuket has a tropical monsoonal climate and thanks to its position in the Andaman Sea, temperatures remain warm all through the year. Afternoon temperatures during high season are usually around 30°C-33°C meaning that a splash in the pool or a swim in the sea is always pleasant.
Phuket is an all year-round holiday destination but there is a "dry season" when you will experience consistently fine weather, and perhaps therefore the best time to visit is from November through to April. This is high season with low amounts of rainfall, calm seas, warm temperatures with plenty of sunshine.
However, good weather is possible at any time of year and a family holiday in Phuket during the low season (April - October) can provide a wonderful tropical vacation at a lower price. The chance of a downpour is higher but rainfall usually comes in short, heavy bursts in the afternoons.