The dating of Easter today follows the same. Accordingly, churches in the West observe it on the first day of the full moon that occurs on or following the Spring equinox on March 21., it became a movable feast between March 21 and April 25.
Still some churches in the East observe Easter according to the date of the Passover festival.
The preparation takes off as early as on the Ash Wednesday from which the period of penitence in the Lent begins. The Lent and the Holy week end on the Easter Sunday, the day of resurrection.