Because Easter is a Christian holiday, you might be expecting it to have a particular date like Christmas. Actually, Easter is determined by the Jewish calendar.
This is actually because in the Bible, the death, burial, and resurrection of Jesus Christ happened after the Jewish festival of Passover, and the followers wanted it to be celebrated as such.
From the New Testament, we know that the death and resurrection of Jesus happened around the time of the Jewish feast of Passover. So according to the Gospels of Matthew, Mark and Luke, the last meal Jesus shared with his disciples was a Passover meal, while the Gospel of John says that Jesus died on the feast of Passover itself. In those days, the Jews had the Passover celebration on the 14th day of the first month which is in accordance with the Bible’s commands on Lev. 23:5, Num. 28:16, and Josh. 5:11.
The months of the Jewish calendar each started at new moon, so the 14th day would be the day of the full moon. The first month is called Nisan, and it was the month that began from the spring new moon. In other words, the Passover was celebrated on the first full moon which follows the vernal equinox and was therefore a movable feast.
So during the 4th century, it was decided that Easter would fall after the first full moon that follows the vernal or spring equinox. (The equinox is a day in the year on which daytime and night-time are of the same length. And this happens two times a year, once in spring and once in autumn.)
Ever since then, the Passover full moon or Paschal full moon has been determined from the historical tables, and it is the first full moon date after March 20. And therefore, Easter is always celebrated on the Sunday immediately following it.
So according to the Metonic cycle, the Passover Full Moon falls on a recurring sequence of 19 dates which ranges from March 21 to April 25. That’s why the date for this year’s Easter is on April 21. And it’s still in range.