quinquennium: A practically useless word referring to a period of five years. There are twenty quinquennia (or, if you're boring, quinquenniums) in a centennial.
Few things are worth celebrating after only five years. Only three come to mind at the moment: surviving cancer, staying sober, and being divorced. I've never had cancer, and I'm not a huge fan of staying sober, and I still have another year and a half before the quinquennial of my divorce. Doesn't mean I can't start celebrating now, though, does it?
Is there something you celebrate after only five years?