Both traditions follow the same rule from the First Council of Nicaea in 325: Pascha falls on the first Sunday after the first full moon on or after the vernal equinox. They diverge on two points of reckoning, not on the rule itself.

The calendar. The West reckons from the Gregorian calendar of 1582. Most Orthodox churches reckon the Paschalion from the older Julian calendar, now thirteen days behind, so the Julian “March 21” equinox anchor falls on April 3 by civil reckoning.

The moon. The Gregorian reform also corrected the tables predicting the paschal full moon, which had drifted several days off the true astronomical moon. The Julian Paschalion retains the older nineteen-year cycle.

Because both always land on a Sunday, the gap is always a whole number of weeks. A further constraint on the Orthodox side requires Pascha to follow Passover, which is why the gap sometimes widens to a full month.