Isn't Easter late this year? Well no, actually it isn't. Easter falls in the second half of April about 25% of the time, so Easter 2006 is quite normal. It fell as late as April 20th three years ago, and on April 23rd in 2000. If you want a really late Easter you need to wait just five years, because Easter 2011 falls on April 24th (which is stunningly rarely late). It can even fall as late as April 25th, but that only happens about once a century (and if you weren't around in 1943, you'll have to hang on until 2038 to experience it).
Easter Day always falls "on the first Sunday after the first full moon after the spring equinox". This year the springequinox was way back on March 20th but the first full moon wasn't until last Wednesday, and that makes Easter Day tomorrow. Simple (ish). Except that the church doesn't use the real spring equinox, it uses the date of the spring equinox in 325AD. And the church doesn't use the real full moon, it uses an approximate one called the Paschal Full Moon, with dates that repeat every 19 years. If (and only if) this Paschal Full Moon falls on Sunday April 18th, then Easter Day can be as late as Sunday April 25th. I'll leave the full explanation to others (here), but the upshot is that Easter Day can fall on any date from March 22nd to April 25th inclusive.