The best time to go to Sayulita is November, when it's not as crowded as during winter but the weather is drier and temperatures start dropping a little.
Right before November except a fair amount of rain and high temperatures, in the mid 80s.
Winter in Sayulita is the peak season, especially during the holidays, when you can easily pay more than $200/night for a hotel in Sayulita. If you don’t like crowds we recommend you avoid December. If you want to spend as little as possible and don't mind the tropical very humid weather, then travel to Sayulita during the summer, prices on accomodations drop quite a bit during this season.