Most people know that "High Season" (or "Peak Season"), when the rates are the most expensive, is in the Winter. The Christmas Holidays may be considered the beginning of the Peak season, but they are usually less expensive than the true "High Season" which is from Mid-January through April. Most Resort Condos lower their prices in May.
Many folks think that the Summer months are "off-season" but that is not really true. Florida families and families from all over the USA love to come to Florida in the Summer for their "Family Vacation". Siesta Key is packed all summer, until school starts in late August.
Low Season is September and part of October. By mid-October European's (for whatever reason) flock to Siesta Key and Thanksgiving is very popular.
The very best deals may be found the last week in August, all of September, and the first 2 weeks in December. Otherwise, plan on pretty much paying "List Price".