how and when it is celebrated

You have probably noticed that the date for Resurrection Sunday ("Easter") varies considerably from one year to the next.
This chosen date comes from a formula established by Roman Emperor Constantine (Flavius Valerius Constantinus (February 27, 272–May 22, 337)), commonly known as Constantine I or Constantine the Great, and the Council of Nicaea in Bithynia (now Isnik, Turkey) in A.D. 325.

The Council of Nicaea was the first ecumenical council in the history of the church which determined that Easter should be celebrated on the Sunday following the first full moon after the March equinox after the Jewish Passover.

Practically:
find the first day of spring (about March 21-22), on a calendar that lists basic astronomical data. Then look for the next full moon, usually indicated in a corner box of the calendar. Resurrection Sunday will then fall on the following Sunday.

Constantine was the author of other changes too:

a) By a decree in 321, Constantine announced that the courts to be closed on the venerable day of the sun (Sunday), and introduced the seven-day week into the Julian calendar. The weekdays had no names but were known only by numbers. The week displaced the old Kalends, Nones and Ides, and eventually the days acquired the names of the seven heavenly bodies--Sun, Moon, Mars, Mercury, Jupiter, Venus and Saturn.
b) By the Edict of Milan in 313 (initiated by Constantine in the West, but agreed to by Licinius in the East), Constantine instituted freedom of religion. This had the effect of legalizing Christianity. The Church did not become the official state religion of the Roman Empire at that time. Persecution of Christians by the Roman Empire in any form from 313 ended.
c) He made December 25th, the birthday of the pagan Unconquered Sun god, the official holiday it is now the birthday of Jesus. It is likely that he also instituted celebrating Easter and Lent based on pagan holidays.


Find the date of Resurrection Sunday for any year (1900-2299).