The Florida oneway car rental deals usually take place in the fall. For this particular kind of oneway car rentals, there are no real set dates every year. It can vary by a week or two, depending on the rental volume in in the north vs the projected demand for more cars in Florida for the winter months.
That being said, those Florida one way specials tend to take place during October and November; you can start watching for the deals to be advertised usually by the end of August/beginning of September.
In order to be up-to-date about when car rental deals going to Florida will start, I would recommend going as many car rental company websites as you can and signing up to receive email updates of upcoming Car Rental Deals and one way car rental specials.
That way you'll be sure to have plenty of notice for the Florida one way deals when they happen (availability is limited), as well as for any other car rental specials that are offered.