The date is movable because it depends on astronomical events that shift.
The rule is that Easter is the Sunday after the full moon that is next after the March equinox. … It assumes that the equinox is on March 21 … Palm Sunday <Passover
Vernal Equinox
Full Moon
Passover
Lazarus Saturday
Palm Sunday
Easter
Lazarus Saturday in the Eastern Orthodox Church and Oriental Orthodoxy is the day before Palm Sunday to which it is liturgically linked. It celebrates the raising of Lazarus of Bethany, the narrative of which is found in the Gospel of John. Wikipedia
Date: Saturday, April 20, 2019