Easter is on the first Sunady after the full moon after the Vernal Equinox (aka the first day of spring)
This year Easter is on March 31st!
The vernal equinox was on March 20th. The next time ther will be a full moon is March 27th and the very next Sunday after that full moon is March 31st!
And then you count back forty days, but don't count sundays and that is when Ash Wednesday is!!
That's why Easter Sunday coul be as early as March 21st or as late as April 21st. I may have these dates off by a few!
It all depends on when the full moon occurs after the vernal equinox (first day of spring)!
Of course if there's a leap year (a year with february having 29 days) then the vernal equinox will be on March 19th!