Robin, welcome to the PAD club. If there are any rules posted anywhere I'm not aware of them. A PAD project is a fairly personal thing and any rules you want to go by will be imposed by yourself for the most part. In keeping with what I believe was the original intent of PAD you are 'expected' to take and post one image each day. However, everyone realizes that there will be days when one cannot take an image. It appears that the great majority of PAD'ers accept the fact that some daily entries will have been taken on another day. We all get sick, have camera or computer trouble, deal with family and work emergencies, etc. Some folks go days without making a post and then catch up. My personal rule is to try my best to take and post a new image each day but if I can't do that I don't have a problem with posting an image I took on another day. In the end you are doing this to have fun and expand your photographic skills. As long as you are happy with what you are doing and make a reasonable effort to keep in mind what "PAD" stands for you'll be fine. The PAD club certainly won't send a posse out for you if you stray a little. lol
And one more thing.....unless you really want to burden yourself down DON'T think you have to post a masterpiece every day.
http://www.pbase.com/ralph