Easter is always the 1st Sunday after the 1st full moon after the Spring Equinox (which is March 20). This dating of Easter is based on the lunar calendar that Hebrew people used to identify passover, which is why it moves around on our Roman calendar.
 
Here's the interesting info. This year is the earliest Easter any of us will ever see the rest of our lives! And only the most elderly of our population have ever seen it this early (95 years old or above!). And none of us have ever, or will ever, see it a day earlier! Here's the facts:
 
1) The next time Easter will be this early (March 23) will be the year 2228 (220 years from now). The last time it was this early was 1913 (so if you're 95 or older, you are the only ones that were around for that!).

2) The next time it will be a day earlier, March 22, will be in the year 2285 (277 years from now). The last time it was on March 22 was 1818. So, no one alive today has or will ever see it any earlier than this year!