Easter or Resurrection Sunday commemorates Jesus' resurrection from the dead, a Christian celebration and holiday mentioned in the New Testament as having happened the third day after his death by the Romans at Calvary. It is a 40-day cycle of fasting, prayer, and penance at the end of the Passion of Jesus.
Meanwhile, Christmas is an annual holiday that commemorates the birth of Jesus Christ, which was celebrated overwhelmingly by billions of people all over the world as a religious and cultural festivity. It is a national holiday, observed both religiously by the majority of Christians and traditionally by many non-Christians in many countries worldwide, and an important part of the surrounding holiday season.
Considering these two holidays take place every year, one might wonder why Easter changes its date but Christmas doesn't. Christmas is celebrated on a set date, on December 25 in the Catholic Church, or January 7 in the Orthodox Church. Yet both Christian celebrations are combined with earlier pagan festivals, so why is one fixed and the other not?
Why does Easter change but Christmas doesn't?
Easter Day must always be on Sunday and it is set on the first Sunday after the first full moon, or after the vernal equinox which happens on March 21. If the Full Moon is on a Sunday, the next Sunday is Easter.
The Easter Day holiday observes similar rules as Passover day and is decided based on these rules. The death of Jesus on that day is marked by his relation of death to the Passover lamb sacrifice.
Not least through efforts to harmonize solar and lunar calendars, the date of Easter fluctuates, and the problem becomes much more complex when different strands of Christianity are using various formulas for their estimation. If Easter had been on a set date, Good Friday would not be on a Friday and all similar holidays would not be on the same day.