I flew SW out of BWI last year leaving the Monday before LD and returning the Sunday of Labor Day weekend. First - I booked my return Sunday flight at $79. Then I waited until my out bound flight was $69 and booked that. Then they offered a $59 internet special, so I rebooked both flights and took the credit. Finally, about 2 -3 weeks before we left, I rebooked the outbound flight with a ding for $49. I couldn't get a ding on the return flight.
So - to answer your question - SW does have cheap flights during that time period, but you have to wait and wait to get them. It was a pain, honestly, but I was determined.
I ended up with a $120 credit, which I already applied to flights to Denver over spring break.
I am going again this year - returning Saturday. Once thes $79 each way comes out, I will book and I just might stick with it there since I am going Sunday to Saturday.
I have flown over LD several times. Generally the cheap fares come out in July after the 4th. This past year was just a very slow wait!