Easter always occurs on the first Sunday after the Paschal Full Moon (the first full moon that occurs after the vernal equinox, which signifies the beginning of spring in the northern hemisphere), according to The Old Farmers Almanac.

How do they figure out Easter every year?

The simple standard definition of Easter is that it is the first Sunday after the full Moon that occurs on or after the spring equinox. If the full Moon falls on a Sunday then Easter is the next Sunday.