As per the Gregorian calendar, what's the earliest date in March on which Easter Sunday can possibly fall?

Easter is a moveable holiday, this means that its date varies from year to year. In Western churches, like the Roman Catholic Church, which uses the Gregorian calendar, the earliest possible date is March 22, while the latest is April 25. However, these are very rare — the next time Easter falls on April 25 is in the year 2038, and the next time the date is March 22 will be in the year 2285. The most common date between the years 1900 and 2100 is April 19.

Eastern churches, including the Eastern Orthodox Church, use the Julian calendar, therefore the date usually differs from the Gregorian calendar, even though the dates are computed in a similar way.

The date of Easter is determined as the first Sunday after the Paschal full moon falling on or next after the spring equinox (March 21 — a fixed date not the real, astronomical equinox). The Paschal Full Moon is the first full moon after the spring equinox. Also called the “Egg” Moon, this moon sometimes occurs in March and sometimes in April. The word “Paschal” means “Passover” in Greek (a transliteration of the Hebrew word “Pesach”).

The first Christians followed the biblical Hebrew calendar and timed the observance of Easter in relation to the date of the Jewish Passover or the Jews' deliverance from slavery in Egypt. In order to differentiate between Christians and Jews, the First Council of Nicaea (325 AD) established an independent and uniform calculation of the Easter date which has been in use until now.