"Forasmuch as there are certain persons who kneel on the Lord's Day and in the days of Pentecost, therefore, to the intent that all things may be uniformly observed everywhere (in every parish), it seems good to the holy Synod that prayer be rendered unto God standing." (Canon XX of the Ecumenical Council of Nicaea, A.D 325).
"We consider it forbidden to pray on bended knees on the Lord's Day." (Tertullian, 210).
"There are many other observances in the Church which, though due to Tradition, have acquired the authority of the written Law, as, for instance, the practice of not praying on bended knees on Sunday." (St Jerome, 330).
Eastern Christians do not kneel to pray in Church during the Easter Season.