Source: Soul Food: Notes for Living our Christian faith in modern times

We should not kneel or make prostrations (poklony) from Easter Sunday (Pascha) to Pentecost. Kneeling and prostrating are postures of repentance for our sins. The brilliant solemnity of Christ’s Resurrection, which we celebrate for forty days, takes precedence over external manifestations of penitence. For these same reasons, we do not generally kneel on any Sunday of the year, as each Sunday is a “mini-Pascha.” We resume kneeling at the “Kneeling Vespers” celebrated on the evening of Holy Pentecost.