Fr. Z, in his excellent blog, makes many fine arguments for this return. We base this beautiful Solemnity on the final Gospel accounts. The evangelists tell us that 40 days after the Resurretion, the Lord appeared to the surviving 11 Apostles for the last time, charging them to preach the Good News to all nations and baptize everyone in the Name of the Father, the Son and the Holy Spirit. He then promised to be with them (and us) always, even to the end of time.
The number 40 has a special biblical significance. In the book of Genesis, we read that the Lord sent a deluge upon the earth for 40 days and 40 nights to purify creation. Ancient Israel wandered the desert for 40 years. Forty days after the birth of Christ, the Blessed Mother and St. Joseph took the holy Infant to the Temple to present Him to the Lord. Jesus spent 40 days and 40 nights in the desert after His baptism in the Jordan by St. John the Baptist.
There is also another dimension that we might want to explore, the notion of the significance of Thursday. Let us look at Jesus' promise, "Lo, I am with you always, even to the end of the age." Jesus was about to leave His beloved Apostolic band, and yet, He promises to remain with them. How can this be? We need to go back 43 days to another Thursday, Holy Thursday, wherein the Lord instituted the Sacrament of the Most Holy Eucharist. When Jesus spoke the words, "This is My Body" and "This is My Blood", He meant what He said. This was how He intended to remain with us, under the forms of bread and wine.
If we look at nearly every post-Resurrection appearance that the Lord made throughout these 40 days, each of them had some element of the Eucharist. Even his appearance to St. Mary Magdalene carries with it a Eucharistic reference. When He tells her to stop hanging on to Him, He is trying to prepare her for the fact that He will take on a new form, that of the Eucharist. When He meets up with the two disciples walking along the road to Emmaus, He celebrates the Eucharist with them, breaking both the Scriptures and the Bread, helping them to see that this is how He will remain with them.
While Sunday is the day of the Lord, we should not lose that important connection to Thursday. Holy Thursday points the way to Ascension Thursday.