Calendars printed in the United States put Saturday as the seventh day, but in Europe and other places in the world, Sunday is shown as the seventh day. So how can we be sure?
The seven-day weekly cycle is not tied to any patterns or alignments of the sun, moon or stars. It’s a non-stop serial counting of days, one after the other. Scripture tells us the cycle was established by God. After six days of creation, God rested on the seventh day (Genesis 2:1-2 Genesis 2:1-2  Thus the heavens and the earth were finished, and all the host of them.
 And on the seventh day God ended his work which he had made; and he rested on the seventh day from all his work which he had made.
American King James Version×). Since then, the counting off of days in groups of sevens—with each group called a week and every seventh day designated as a day of rest—has continued unbroken up to our own time.
English-speaking countries call the seventh day Saturday, but the word for that day in many languages indicates its origins as the biblical day of rest. For example, the seventh day in Spanish is called Sabado (coming from the Hebrew shabbat). This too is made plain on printed calendars in those languages.
Could we have lost our place?
If you have ever tried to count off numbers in a sequence you know how easy it is to lose your place. Could this have happened with the counting of the seventh day? After all, it's been a very long time since God established the first day of rest.
If the sequence had gotten all jumbled up sometime between the first day of rest and the Exodus of Israel out of Egypt, the true seventh day was clearly reestablished by the time the Ten Commandments were given.
We can conclude confidently that our current Saturday is the seventh day of the week, on the same weekly cycle that has been in place since the beginning of creation—a cycle established and confirmed by God Himself.
In the wilderness, God fed the people of Israel with manna. The manna would appear on the ground every day except the seventh day, the day of rest (Exodus 16:14-30 Exodus 16:14-30  And when the dew that lay was gone up, behold, on the face of the wilderness there lay a small round thing, as small as the hoar frost on the ground.
 And when the children of Israel saw it, they said one to another, It is manna: for they knew not what it was. And Moses said to them, This is the bread which the LORD has given you to eat.
 This is the thing which the LORD has commanded, Gather of it every man according to his eating, an omer for every man, according to the number of your persons; take you every man for them which are in his tents.
 And the children of Israel did so, and gathered, some more, some less.
 And when they did mete it with an omer, he that gathered much had nothing over, and he that gathered little had no lack; they gathered every man according to his eating.
 And Moses said, Let no man leave of it till the morning.
 Notwithstanding they listened not to Moses; but some of them left of it until the morning, and it bred worms, and stank: and Moses was wroth with them.
 And they gathered it every morning, every man according to his eating: and when the sun waxed hot, it melted.
 And it came to pass, that on the sixth day they gathered twice as much bread, two omers for one man: and all the rulers of the congregation came and told Moses.
 And he said to them, This is that which the LORD has said, To morrow is the rest of the holy sabbath to the LORD: bake that which you will bake to day, and seethe that you will seethe; and that which remains over lay up for you to be kept until the morning.
 And they laid it up till the morning, as Moses bade: and it did not stink, neither was there any worm therein.
 And Moses said, Eat that to day; for to day is a sabbath to the LORD: to day you shall not find it in the field.
 Six days you shall gather it; but on the seventh day, which is the sabbath, in it there shall be none.
 And it came to pass, that there went out some of the people on the seventh day for to gather, and they found none.
 And the LORD said to Moses, How long refuse you to keep my commandments and my laws?
 See, for that the LORD has given you the sabbath, therefore he gives you on the sixth day the bread of two days; abide you every man in his place, let no man go out of his place on the seventh day.
 So the people rested on the seventh day.
American King James Version×). This sequence would repeat every seven days, over and over for 40 years. The weekly cycle of manna only stopped when Israel entered the promised land. Four decades of repetition is plenty of time to clearly establish which day was the day of rest.
If the Sabbath had somehow been lost between creation and the exodus, 40 years of weekly miracles would have definitely reestablished it.
Did Israel ever lose track of the seventh day?
The Exodus was still a very long time in the past. Plus Israel went through many periods of turmoil, forgetfulness of God, invasion, deportation, etc. Could Israel—and later the Jewish people—have mixed up the sequence of counting the seven-day weekly cycle through all that?
Jesus Christ, the very Word of God, put off the glory He shared with the Father and was born a flesh and blood human being. He was the “Lord of the Sabbath” (Matthew 12:1-8 Matthew 12:1-8  At that time Jesus went on the sabbath day through the corn; and his disciples were an hungered, and began to pluck the ears of corn and to eat.
 But when the Pharisees saw it, they said to him, Behold, your disciples do that which is not lawful to do on the sabbath day.
 But he said to them, Have you not read what David did, when he was an hungered, and they that were with him;
 How he entered into the house of God, and did eat the show bread, which was not lawful for him to eat, neither for them which were with him, but only for the priests?
 Or have you not read in the law, how that on the sabbath days the priests in the temple profane the sabbath, and are blameless?
 But I say to you, That in this place is one greater than the temple.
 But if you had known what this means, I will have mercy, and not sacrifice, you would not have condemned the guiltless.
 For the Son of man is Lord even of the sabbath day.
American King James Version×; Mark 2:23-28 Mark 2:23-28  And it came to pass, that he went through the corn fields on the sabbath day; and his disciples began, as they went, to pluck the ears of corn.  And the Pharisees said to him, Behold, why do they on the sabbath day that which is not lawful?  And he said to them, Have you never read what David did, when he had need, and was an hungered, he, and they that were with him?  How he went into the house of God in the days of Abiathar the high priest, and did eat the show bread, which is not lawful to eat but for the priests, and gave also to them which were with him?  And he said to them, The sabbath was made for man, and not man for the sabbath:  Therefore the Son of man is Lord also of the sabbath.
American King James Version×; Luke 6:1-5 Luke 6:1-5  And it came to pass on the second sabbath after the first, that he went through the corn fields; and his disciples plucked the ears of corn, and did eat, rubbing them in their hands.  And certain of the Pharisees said to them, Why do you that which is not lawful to do on the sabbath days?  And Jesus answering them said, Have you not read so much as this, what David did, when himself was an hungered, and they which were with him;  How he went into the house of God, and did take and eat the show bread, and gave also to them that were with him; which it is not lawful to eat but for the priests alone?  And he said to them, That the Son of man is Lord also of the sabbath.
American King James Version×). Jesus would have known if He, or His fellow Jews, had the sequence of the seventh day messed up.
The record of Jesus’ life shows He rested on the seventh day in obedience to the commandment (John 15:10 John 15:10If you keep my commandments, you shall abide in my love; even as I have kept my Father's commandments, and abide in his love.
American King James Version×). Jesus observed the day of rest on the same day of the week as His fellow Jews. The religious authorities of the day may have disputed what types of activities were appropriate for observing the seventh day as a day of rest, but not which day of the week it was.
Jesus' obedience to the Fourth Commandment confirmed the seventh day as kept by the Jews was the correct day.
Did the Jewish people lose track of the seventh day after Jesus’ death?
Years before the time of Jesus’ death, and in the centuries since, the Jewish people have been widely dispersed throughout the Middle East and Europe. Wherever they have gone, they have fiercely guarded the observance of the seven-day weekly cycle so they might never break the Fourth Commandment.
It's possible to consider that isolated pockets of Jews may have lost track of the weekly cycle due to some local turmoil, but such an error would have quickly been corrected by comparing notes with the many other Jewish communities in other countries and continents—communities which had not experienced any interruption or disruption in the seven-day weekly cycle.
However, there isn’t any historical record of any dispute among any Jewish groups that the day we call Saturday is the seventh day of the weekly cycle, God’s day of rest. The Jewish people have successfully kept the original seven-day weekly cycle intact.
Now, one last question that sometimes comes up:
Did the change to the Gregorian calendar in 1582 lose track of the seventh day?
From 46 B.C. up to A.D. 1582, the Western world used what is called the Julian calendar. It was a vast improvement over the previous Roman calendars because it finally kept the months in sync with the seasons. However, after many centuries even the Julian calendar was about 10 days out of sync with the solar system.
In 1582 the old Julian calendar was replaced with the new, improved Gregorian calendar, which is the calendar system we still use today. At the time of the change to the new calendar system, those 10 days were simply dropped out of the calendar to get the dating system of the calendar back in sync with the solar system. It was Thursday, Oct. 4, 1582, and the next day, Friday, should have been Oct. 5. However, the new calendar made the next day Oct. 15 instead.
That change had no impact on the seven-day weekly cycle. Whether that following Friday was numbered as the 5th of the month or the 15th of the month, it was still the sixth day of the week, and the day that followed was still the seventh day of the week.
Even today we regularly add a day into our calendar every four years. We call it a “leap year.” Changing the number of days in a calendar month does not alter the weekly cycle—there is no six-day week, or eight-day week.
So we can conclude confidently that our current Saturday is the seventh day of the week, on the same weekly cycle that has been in place since the beginning of creation—a cycle established and confirmed by God Himself.