The last day of the week is the Sabbath Day in the Jewish religion and is observed from sunset on Friday to sunset on Saturday. It is a day of rest and no unnecessary work is allowed to be done in honor of the Commandment to keep the Sabbath Day holy.
Jewish Christians, like the Apostles and early converts, observed the Sabbath regulations and also celebrated the “breaking of the bread” (the most ancient term used for the Mass or Eucharistic Liturgy) the following day, Sunday, to honor the Resurrection of Christ on Easter Sunday.
When more Gentiles came into the Church, and after the expulsion of Christian Jews from Judaism (in response to the destruction of the Temple of Jerusalem in 70 AD by the Roman Emperor Titus), Christianity as a separate religion abandoned the Jewish observance of Saturday Sabbath. Sunday worship of God replaced it, since it was considered the “day of the Lord” (day of Resurrection), and, in fact, in many languages the word for Sunday means “day of the Lord” (e.g., in Italian, domenica; in Spanish, domingo; in French, dimanche) whereas Saturday is translated as sabbath (Italian: sabato; Spanish: sábádo; French: samedi).
Christians maintained the tradition of going to church on Sunday as it is the “day of the Lord,” but even when the Julian Calendar (devised by Julius Caesar in 45 BC) was replaced by the Gregorian Calendar (by Pope Gregory XIII in 1582 AD) the first day of the week remained Sunday, and Saturday remained the end of the week, the Sabbath day.
Catholics, can, however, go to church on Saturday evening after sunset and have it count as going to church on Sunday, since liturgical time is based on the Hebrew practice of counting from sunset to sunset rather than from midnight to midnight.