Easter is a movable holiday so the date in which it is celebrated changes every year. This can be confusing, especially if you don’t know the rules for calculating the date. It was the First Council of Nicaea (325) that agreed Christians should at least have a consistent way to determine the holiday, but it took several more centuries to arrive at our current means of calculating the date using a lunisolar calender cycle. Officially, Easter falls on the first Sunday following the the 14th day of the lunar month (which is essentially a full moon) occurring after the spring equinox. It always falls sometime within the range of March 22nd and April 25th.
What’s an equinox - An equinox is a day of the year when day and night are approximately the same length in time. This occurs twice a year on the first day of spring and fall. In the northern hemisphere, the spring equinox technically occurs on either March 20 or 21st, however the Church recognizes the 21st as the official spring equinox.
Why it’s late this year - This year, the full moon in March fell on March 19th, just two days before the spring equinox. The next full moon does not occur until Monday April 18th. The next Sunday following that is on April 24th, otherwise known as Easter 2011.