Why is Easter a floating holy day? Why can’t the Church celebrate Easter on the same Sunday each year? The Bishops have moved other Holy Days, for convenience, why not Easter?
The date of Easter varies each year because it is linked to the cycle of the Moon, relative to the cycle of the Sun. In order to set the date of Easter one must first look for the vernal (spring) equinox, which is March 20. The word “equinox” refers to that time when the length of day and night are equal. It is also the date we set for the official beginning of spring.
Having set our sights on March 20, we next look for the first full moon following March 20. Some years, the first full moon occurs quickly, within days of the equinox. Other years it occurs weeks later.
For the Jewish people, this first full moon after the equinox also signaled Passover. And since it was at the Passover feast that our Lord Jesus suffered and died and rose, we Christians always fix Easter to coincide with Passover.
So then, Easter, (which is always on a Sunday since Christ rose on the first day of the week), is celebrated on the Sunday following the full moon after the vernal equinox.
Historically there were great debates within the Church in the East and the West about setting the date of Easter. The system described above was finally settled upon. But today, we still find that the exact date for Easter varies a bit in the western and eastern parts of the Church since many of the Eastern rites still use the more ancient Julian calendar, rather than the Gregorian calendar used by the church in the West.
Your wish for a fixed Day for Easter, as is the case with Christmas and other feasts, is understandable. But as you can see, the relationship of the Moon relative to the sun doesn't fit perfectly into our modern systems of timekeeping and to fix the date as you suggest would probably open old debates that caused great harm in the early Church.
"Building our Catholic faith one question at a time."