I'll preface this by saying that I'm not a PHP coder and this is my first attempt at this, so bear with me. Also, I've searched the forums and couldn't find anything that would do what I wanted. Feel free to direct me, but for now I think I've figured out how to get what I want done on my own.
Basically, I've got an image at the top of my page that I wanted to have dynamically change depending on the date. For example, on December 25th, I'd want the image to be Christmas-themed, on January 1st, it'd be New Year's themed. I wanted this all done dynamically, so that at 12:01AM it would just change on it's own without any need for me to get involved. Basically I threw together some simple "if/elseif" statements that did what I wanted. I think you could use it for any kind of date based, HTML formatted information though, not just page headers and it could possibly run as a plug-in, but that's beyond my knowlege right now. You could probably also put it in a separate file and use a PHP include if you wanted. I'm just posting this for those PHP newbies like myself that can use all the help they can implementing neat ideas.
Regardless, the information is at my site: http://www.playswellwithothers.net/index.php/2005/12/01/date-specific-page-headers/ so if anyone finds this remotely useful or interesting, feel free to head on over.