At least from Detroit, Spirit's "special" fares generally aren't valid for weekend travel (departing Friday or Saturday & returning Saturday or Sunday). Sometimes you may find one of those days available, but often the cost of flying in the other direction kills the deals. There have been a few exceptions, however -- for example, I scored a "-2" fare to NYC departing Saturday AM and returning Saturday PM in January (total was about $20).
If you're flying out of Detroit and you're looking for good deals to Orlando, sign up for Southwest's "Ding" program. They've often (like two or three times per month) had DTW/MCO for OW$25-$50 (including weekends!!!) since spring... probably because their nonstop service is doing poorly -- I've utilized it four RT since June (soon to be five) and most of the flights have been EMPTY.