About not needing (may de-een) kneeling, on Sundays and in the 50 days of Eastertime:

There are some people who kneel down on Sunday and during the Easter season, the fifty days from Easter Sunday to Pentecost Sunday.  Therefore, it has pleased the holy Council to decree that people should offer their prayers to the Lord, standing.  This is required so that in each diocese (en pase paroikia) everything will be done in harmony (omoiohs).

(Canon 20 of the Council of Nicea, 325 A.D., binding on the whole Church)

We consider it unlawful to fast or to pray kneeling, on the Lord's Day.  We enjoy the same liberty from Easter Sunday to  Pentecost Sunday. 

(Tertullian, De Corona Militis, s. 3, 4.)

Augustine and others give the reason for this tradition.  They say that we commemorate the Resurrection of Christ. Standing on Sunday and in Eastertime signifies the rest and joy of our own resurrection, assured by the Resurrection of Jesus Christ.

(Literally:) About  the not needing, on sundays and the of fifty days, by knee bending:

Since some are on the sunday by knee bending and in the of the fifty days, about the all things in every diocese harmoniously to be carried out, standing it has pleased the holy synod the prayers to be offered to the Lord.

(Decrees of the Ecumenical Councils, Vol. I.  Edited by Norman P.Tanner, S.J. Washington, D.C.: Georgetown University Press, 1990, p. 16. Texts of the Council of Nicea are also available online.)