Good Friday, meanwhile, like many of the other most important Christian holidays, is a set number of days before Easter. The only problem is that the date of Easter is probably the most complicated celebratory calculation this side of Hinduism, which has a number of competing religious calendars. The standard rule is "the Sunday after the first full moon on or after the day of the vernal equinox." But in fact, the actual divination of the date is so involved that it has its own offical name: "computus." And so challenging that Carl Friedrich Gauss, one of history's greatest mathematicians, devoted the time to create an algorithm for it. It goes on for many lines. You can look it up. And, of course, it doesn't work for Eastern Orthodox Easter (about one month later than the Western Christian one this year, on April 27).
Unfortunately, "computus" isn't in Dictionary.com, but it is in Wikipedia.