I booked a flight to Sanya on June 28th, but the weather forecast said it would rain.

If the weather is bad and the flight is unsafe, the airline will cancel the flight in advance, and then you can change the ticket or refund the ticket. If it is a normal flight, it means there is no problem. As for the rain after you arrive in Sanya, don't worry. Rainy days are a rainy scene, a different scene, and you will be in a good mood from another angle. Have a nice trip!