November to March is Thailand’s easiest all-round window: lower humidity, calmer island weather and comfortable city evenings. It is also peak season, so flights and strong hotels reward early decisions.
For Bangkok + Phuket or Krabi
December through February offers the most dependable combination. March and April are hotter but can still work if you plan pool time and avoid stacking outdoor attractions through the afternoon.
The Andaman coast sees more rain from roughly May to October. Showers do not ruin every day, but boat schedules and sea conditions can change, so a flexible plan matters.
For Koh Samui
Koh Samui follows a different pattern from Phuket. The middle of the year can be a good island window, while heavier rain often arrives later in the autumn. That difference is why “Thailand in monsoon” is not a complete answer.
For families and first international trips
Four to six nights is enough for Bangkok plus one coastal base. Keep arrival day gentle, reserve one open half-day and check whether transfers are private or shared. Small logistical choices do more for comfort than one additional attraction.
Before you book
Entry rules, airline schedules and local fees can change. Reconfirm the current requirements for your passport and travel dates, then price baggage, transfers and activities into the complete trip rather than comparing hotel-only figures.