At the beginning, the Church, departing from Jerusalem, used to celebrate the Christian Passover, the Feast of the Lordly Resurrection three days after the Jewish Passover, Pessach, three days after the 14 of Nissan. Needless to say that every Sunday is the weekly celebration of Christ's Resurrection.

Soon, especially in the Nicea Council (325 A.D.) the Church was keen on distancing itself from the Jewish Passover and Calendar for the following reasons (at least):

1- the Jewish calendar- actually the Jewish year - is incomplete. As a matter of fact a second month of "adar" is added every seven years or so.

2- the Church wished that the yearly celebration of the Lord's Resurrection be always ON A SUNDAY, being "the first day of the week" (in Hebrew : yom rishon) of the new creation, and in Greek "He KYRIAKE", "the day of the Lord" par excellence. (In Armenian : Gyragy').

So, the Nicea Fathers decided that the "Sunday of the Resurrection" (as the name "Easter" is used only in Anglo-Saxon countries, since the eighth century A.D.) BE THE FIRST SUNDAY AFTER THE EQUINOX OF THE SPRING, or : first Sunday after the FULL MOON of the Spring , namely the first Sunday after the 21st of March. And if the equinox-full moon on the 21st of March, Easter should be celebrated on the following Sunday.

As Jesus fasted for FOURTY days, and as He asked us to follow His example, in general, and as He prophesied (Mt 9 : 14 - 15 and //) that his disciples, people of the Wedding, would mourn and fast while the "Bridegroom will be lifted up from them" (on the Cross), the Church started with a three day fasting, the holy triduum before Easter. But, since Jesus fasted BEFORE His ministry, Lent was fixed after the time of Christmas , Epiphany and the Baptism in River Jordan.(Mt II and IV).

The ROMAN CATHOLIC Church (and many Protestant denominations) start Lent 46 days before EASTER, excluding SUNDAYS from fasting and abstinence (we have six Sundays in this period), and so Lent starts always on a WEDNESDAY, Ash Wednesday.

The Oriental (even Catholic) Churches, have TWO FASTING times in this period : LENT (forty days) + the Holy Week.

Even on Sundays they hold fasting and abstinence- which makes them start Lent ON A MONDAY. Apparently the AMBROSIAN RITE follows this practice also, as Lent starts on the Monday before our Ash Wednesday.

As for the Orthodox Churches, following the Julian calendar - which has a delay of 13 days in relation to the Gregorian one, the Feast of the Resurrection follows the same rule : it should fall on the FIRST SUNDAY after THEIR 21 March (=+13 days), after the full moon (equinox also) of Spring.