Turns out, it's slightly complex. It has to do with moon cycles.
After wigging out a little about this task, an afterthought that should've been easy, I googled for the specific formula so I could begin to express it in code. (I had already done a 20, 30 minute search for an existing PHP Class to handle all dates and was unsuccessful.)
What I found next nearly blew my mind. http://php.net/manual/en/function.easter-date.php
This at once delighted and disgusted me. A perfect example of the utility AND horror of PHP's large standard library.