The revelation that the Sabbath was a binding command and a specific period in the seven day weekly cycle was proclaimed before Israel arrived at Mount Sinai and agreed to the Old Covenant in blood. Notice Exodus 16, in which God stated that he would rain bread (manna) from heaven to feed his people in order to test them.
And the people shall go out and gather a certain amount every day, that I may prove them, whether they will walk in my law or not . . . " (Exodus 16:5, HBFV throughout).
From Sunday through Thursday, the people gathered whatever manna they needed. On Friday, when they were to gather twice as much manna as they normally did, Moses stated the following, "This is that which the Lord has said, 'Tomorrow (day seven) is the rest of the holy Sabbath to the Lord"' (Exodus 16:23).
God was both testing the people and training them to know when the acceptable period to keep the Sabbath occurred within the week. On day seven of the week, when no manna fell, some foolishly tried to gather it anyway. The Eternal was not pleased (Exodus 16:28 - 29).
Even after the sins of the people caused them to be punished by wandering the wilderness (Numbers 14:26 - 33) the Eternal, for forty years, continued to feed them on the same weekly cycle. He did not allow the ancient Israelites who left Egypt to worship him during any period they decided. He made sure they understood the acceptable time out of seven for rest and worship.
The fourth commandment
Soon after he began to test the hearts of the people using manna, God gave them the blessing of the Ten Commandments. He left no doubt that his pattern of working the first six days of "creation week" followed by resting on the seventh (Genesis 1 - 2:3) was directly tied to and the foundation of the fourth commandment (see Exodus 20:11).
The New Testament
The Bible tells us that Christ kept the seventh-day Sabbath (Mark 6:2, Luke 4:16, 31, 6:6, 13:10, etc.) as well as the disciples who were with him. Not only did Jesus keep the same time holy as was specified in the Ten Commandments, he claimed to be its Master or Creator (Matthew 12:8, Mark 2:28, Luke 6:5)!
When Jesus gave his famous Olivet prophecy, he made an often-overlooked statement regarding the continuity of the seven-day week and God's rest when he stated the following.
And pray that your flight be not in the winter, nor on the Sabbath (Matthew 24:20).
This statement regarding the end time just ahead of us, made close to 2,000 years ago, is meaningless if the correct time to worship the Eternal would somehow be "lost." Why should any Christians pray this way if, as some Protestants believe, after Jesus' death they would be free to worship on any period in seven they wished? Clearly, Jesus knew that the same Sabbath he kept would not only be known in the End Time, there would also be believers keeping it on the same day he did!
Catholics kept track
We can count on what became the Catholic Church to have kept track of which weekday in seven was the Biblical Sabbath. After the first century A.D. resting and worshipping on a Saturday was equated with something only Jews observed.
The early major Catholic writers were often very anti-Jewish, even virulently anti-Semitic at times, and they did not want to observe the same day that the Jews did. Hence, Catholics did not lose track of time or the weekly cycle, since they knew exactly that they did not want public worship to occur on a Saturday but rather on a Sunday.
The church has admitted which modern weekday the Bible teaches to worship God on versus which time they decided to do it!
"The Catholic Church for over one thousand years before the existence of a Protestant . . . changed the day (for worshipping God) from Saturday to Sunday (Catholic Mirror, September 23, 1893)
"But you may read the Bible from Genesis to Revelation, and you will not find a single line authorizing the sanctification of Sunday . . ." (James Cardinal Gibbons, Archbishop of Baltimore (1877-1921), The Faith of our Fathers)
In 1582 A.D., the Catholics determined that a change was needed in the calendar in order to place it back in harmony with the solar cycle. This fix did not entail changing or interrupting the integrity of the seven-day week that they had kept for more than 1,000 years. Known as the shift from a Julian to Gregorian calendar system (our modern system), Catholics omitted ten dates in October but left the sequence of weekday names intact.
In conclusion
The integrity of the seven-day cycle, created by God himself, has been maintained ever since man began to walk the earth. Time has never been lost. The time the Eternal set apart as holy during "creation week" was the same one kept by Jesus his entire life. This time, called the Sabbath day, runs from sunset on Friday to sunset on Saturday.