Let's start with the ideal - a few references to the canons of the Ecumenical Councils, and one that was accepted by Canon 2 of Penthekte (all quotes taken from the CCEL website):
Canon XX of the Synod at Nicea (First).
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 made to God standing.
Canon XC of the Synod in Trullo (the 5th-6th Synod, Penthekte, or Quintisext).
We have received from our divine Fathers the canon law that in honour of Christ’s resurrection, we are not to kneel on Sundays. Lest therefore we should ignore the fulness of this observance we make it plain to the faithful that after the priests have gone to the Altar for Vespers on Saturdays (according to the prevailing custom) no one shall kneel in prayer until the evening of Sunday, at which time after the entrance for compline, again with bended knees we offer our prayers to the Lord. For taking the night after the Sabbath, which was the forerunner of our Lord’s resurrection, we begin from it to sing in the spirit hymns to God, leading our feast out of darkness into light, and thus during an entire day and night, we celebrate the Resurrection.
Canon XV of Peter Archbishop of Alexandria the Martyr.
Wednesday is to be fasted, because then the Jews conspired to betray Jesus; Friday, because he then suffered for us. We keep the Lord’s Day as a day of joy, because then our Lord rose. Our tradition is, not to kneel on that day.
So ideally, we don't kneel on Sundays because they are triumphant and joyful days. However, kneeling is permitted (and encouraged) on the other days of the week. In the modern days many do kneel on Sundays out of reverence to the Body and Blood of Christ during the epiklesis (the Consecration of the gifts), and the practice continues partially to not squash this reverence, and partially because people do not observe the daily cycle of services - and thus do not see the difference between a weekday and a Sunday.
