Just needed to look that up myself on wikipedia Yes the follow a pattern... but it's not that easy. Everything relies on the Easter Sunday and is then e.g. 3 days before Easter Sunday or 49 days after the Easter Sunday and so on. So far so good, but now it gets a bit tricky... Easter Sunday itself is breaking the scheme a bit, because it's based on spring and full moon - totally weird, but that's how it is.Since I do not want to quote all the stuff, just read yourself over at wikipedia: http://en.wikipedia.org/wiki/Easter#DateSo as long as the dates for Easter Sunday are in the system, the rest is fine, but without Easter Sunday it will be hard (at least for German holidays) to follow a pattern.I don't think that's such a big problem, since you could implement it somehow, but maybe in other parts of the world there are also such "scheme-breaking" holidays which you did or I do not think of, so we here will have a look especially at this point when the devs revisited SLA and give feedback to make it fool-proof ;)