what is the rule for when easter is
Computus For most of their history Christians have calculated Easter independently of the Jewish calendar. In principle, Easter falls on the Sunday following the full moon that follows the northern spring equinox (the paschal full moon). However, the vernal equinox and the full moon are not determined by astronomical observation. The vernal equinox is fixed to fall on 21 March (previously it varied in different areas and in some areas Easter was allowed to fall before the equinox). The full moon is an ecclesiastical full moon determined by reference to a lunar calendar, which again varied in different areas. While Easter now falls at the earliest on the 15th of the lunar month and at the latest on the 21st, in some areas it used to fall at the earliest on the 14th (the day of the paschal full moon) and at the latest on the 20th, or between the sixteenth and the 22nd. The last limit arises from the fact that the crucifixion was considered to have happened on the 14th (the eve of the Passover) and the resurrection therefore on the sixteenth. The "computus" is the procedure of determining the first Sunday after the first ecclesiastical full moon falling on or after 21 March, and the difficulty arose from doing this over the span of centuries without accurate means of measuring the precise tropical year. The synodic month had already been measured to a high degree of accuracy. The schematic model that eventually was accepted is the Metonic cycle, which equates 19 tropical years to 235 synodic months.
Lent Lent (Latin: Quadragesima: Fortieth) is a solemn religious observance in the Christian liturgical calendar that begins on Ash Wednesday and ends approximately six weeks later, before Easter Sunday. The purpose of Lent is the preparation of the believer for Easter through prayer, doing penance, mortifying the flesh, repentance of sins, almsgiving, and self-denial.[1] This event is observed in the Anglican, Eastern Orthodox, Oriental Orthodox, Lutheran, Methodist, and Catholic Churches.[2][3][4] Some Anabaptist and evangelical churches also observe the Lenten season.[5][6] Its institutional purpose is heightened in the annual commemoration of Holy Week, marking the death, burial, and resurrection of Jesus, which recalls the tradition and events of the New Testament beginning on Palm Sunday, further climaxing on Jesus' crucifixion on Good Friday, which ultimately culminates in the joyful celebration on Easter Sunday of the Resurrection of Jesus Christ.