It will usually state like this: all travel involving these funds must be completed by 9/28/11. If so, then you can't fly on 9/29 but you can on 9/28. I had a situation where my credits would expire on 5/12 and we were to return home on the 13th. So I booked as 2 seperate one ways, and used the credits to fly down on 5/6, then paid for the return flight on my cc. That allowed me to use up my credits completely and still keep my desired Sat-Sat vacay.