The answer
The two reasons you have given are correct. We celebrate the Eucharist on the first day of the week, which is Sunday, the day of the Lord, and the day of His Resurrection.This practice was established by the apostles themselves; "Now on the first day of the week, when the disciples came together to break bread" (Acts 20:7).
The writings of the early church Fathers all point to the celebration of the Lord's Day on the first day of the week (Sunday).
Didache: "Christian Assembly on the Lord's Day: 1. But every Lord's day do ye gather yourselves together, and break bread."
The Didascalia: "The apostles further appointed: On the first day of the week let there be service, and the reading of the Holy Scriptures, and the oblation, because on the first day of the week our Lord rose from the place of the dead, and on the first day of the week he arose upon the world, and on the first day of the week he ascended up to heaven, and on the first day of the week he will appear at last with the angels of heaven" (Didascalia 2).
St. Ignatius: "If, therefore, those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord's Day, on which also our life has sprung up again by Him and by His death" (Epistle of Ignatius to the Magnesians, Chapter IX).
Answered by His Eminence Metropolitan Youssef (opens in a new tab)