Depends where you want to go in Thailand.
The rainy season gets to different parts of the country in different months so while it is monsoon in Koh Samui on the eastern side - it can be fine on the western side in Phuket (and vice versa).
The Lonely Planet guides have a section at the front of the book called 'when to go'. This will tell you which parts of the country get hit by the monsoon at the different times of the year. (note; you will nead to read the actual LP Thailand Guidebook - there is an abbreviated section on the LP website but that does not have the level of detail that you are seeking)