I'd been looking for February 2007 and found it open on Sunday, 10/1. (I didn't check on Saturday, 9/30; but I had checked on 9/29.) It appears that they open them up at the 4 month mark. When we booked them for our July, 2006 trip, we did that in May.
We were very pleased with Allegiant. Much less expensive than the majors, and we really liked the nonstop flight. From Lansing, we usually have connections in Detroit, Chicago or Cincinnati.