How is the date of Easter determined every year?

Easter is always the Sunday closest to the first full moon after March 20th.

Passover starts on a specific date of the Jewish Calendar (the 15th of Nisan). The Jewish calendar is a hybrid lunar-solar calendar; every month begins on the night of the full moon. This means that inserting leap days (as the Gregorian calendar does) to keep the schedule aligned with the solar year is impossible. Instead, the Jewish calendar adds an entire leap month every 2 or 3 years (7 out of 19 years).

The current Jewish year (5776) is a leap year. This means that all the holidays are rather late, and Passover doesn't start until April 22. Meanwhile, the first full moon after March 20 is March 27, which is quite early (it could be anywhere from March 21 to April 25).

However, you're right that having almost an entire month between Passover and Easter seems a little wrong—it is! This is because the Jewish calendar doesn't entirely correct for the difference between solar and lunar time-keeping. The Jewish year is slightly longer than the solar year, resulting in a discrepancy of 1 day every 230 years. This means that in the 16 centuries the current accounting of the Jewish year has been in place, the holidays have been gradually getting later and later than they "should" be. Various arguments have taken place regarding what should be done to fix this; unfortunately, as Jews love to argue, we've come to no consensus yet.

I guess that at some point in the next couple decades, most of the global Jewish community will agree to eliminate one of the leap months we would observe, moving all of the holidays a full month earlier (until they start creeping up again).