These are the dates that our management company is using for spring break and summer break. I think the rentals for spring break are coming in more slowly this year. It could be that people feel that there are many places available (and that may well be true given all the new building in the area) so that they have some time to book. People who are pretty flexible with their needs/cost may not hurry with their reservations. People who want certain things (e.g., 3 bedroom, gulf view, specific location) may be more inclined to book early, as would repeat guests.
Spring Break:March 11-April 16
Summer:May 27-August 6
I always come down in early May after spring break and before summer break. Great time to be here and get the places ready for summer guests (and try out new restaurants recommended on this board).
Keep in mind that northern families often have spring break during Easter week (beginning Saturday, April 15) so you may want to consider that when booking your place for yourself (if you want to leave that week open for potential guests). Once northerners become more aware of SoWal, it is likely to extend our peak guest seasons. Students in the north often don't start school until after Labor Day, which leaves several weeks in August available to them for a lovely, uncrowded vacation in SoWal.