if you look under your profile at national.com you will see that the credits have an expiry date, thus it might be that the "free day" will expire at the end of a certain time if it is not used
we have used ours for short trips like the one we took to chicago, we paid 2.54$ total for the two day rental that would have cost approximately 65$ if we had just rented from the website staright up
amazingly, they gave us ONE credit towards our free day for this "rental"