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Christians observe the Lord’s day to keep holy the day our Lord Jesus Christ rose from the dead. For us, this day, Domenica, marks the day of the new creation, when Christ conquered sin, darkness, and death, the day of the new covenant when Christ, the High Priest who had offered Himself as the Unblemished Passover Lamb of Sacrifice on the altar of the cross, gave the promise of everlasting life.

Therefore, Domenica is the fulfillment of the Sabbath of the Old Testament. St. Justin Martyr (d. 165) wrote, “Sunday, indeed, is the day on which we hold our common assembly because it is the first day on which God, transforming the darkness and matter, created the world; and our Savior, Jesus Christ, arose from the dead on the same day.”

Through his death and resurrection, Jesus fulfilled the Old Covenant and instituted the New Covenant – complete with a new priesthood (himself), a new sacrifice (his death on the cross), a “new” law of Grace , and a new day to worship – the day he completed this New Covenant by rising from the dead – Sunday.

Hebrews 7:11-25 records this covenant change clearly: “When there is a change of priesthood (Old Testament priesthood to Jesus), there is necessarily a change of law as well … a former commandment is annulled … (and) a better hope is introduced.”

in Revelation 1:10, St John is “caught up in spirit on the Lord’s day” to be shown the heavenly worship going on. In Acts 20:7, St. Paul and others gather “On the first day of the week … to break bread,” a term used to signify the celebration of the Eucharist (cf Lk 22:19, Mt 26:26). St Paul also asked for collections to be taken when churches gathered “on the first day of the week” (1 Cor 16:2).

Significantly, Jesus’ Emmaus road appearance also takes place “on the first day of the week,” but his disciples don’t recognize him until the breaking of the bread: “he took bread, said the blessing, broke it, and gave it to them. With that … they recognized him” (Lk 24:13-31).

Jesus’ first resurrection appearances to his disciples are on the “first day of the week.” And the great outpouring of the Holy Spirit on Pentecost? On Domenica

If you'd stick to the Word of God instead of the word of man (JustinM), you'd have your facts right. Jesus claimed His day of the week to be the Sabbath (Matt12:8). Of course the Disciples met,prayed and collected tithes and did other stuff on the first day of the week. Why shouldn't they? But did they keep that day holy? No.
The New Covenant nullified the Jewish ceremonial laws meant to point to the Messiahs coming. He was there. When He died there was no further use of it. All that remained of the Laws was the Ten Commandments which was for all people.