I think the rainy season for most of Thailand is officially May to November, with September and October being the wettest months.
Hopefully the sun will be out when I touch down on 29th October, maybe just a last clearing shower
In 2009, I was on Phuket from about Oct 21 through Nov 10. I remember October being pretty rainy.....though it was not constant, and only a couple of straight rainy days in a row. Sufficient sunshine and not at all intolerable.
Then, November came and it was almost miraculous. Mostly sunny (scattered rain, of course), but very nice. I'm hoping for more of the same when i arrive Mid-Nov.
Good luck Patrick! Keep us posted.