Actually, we can be positive that our seventh day is the same day Jesus observed when He was here on Earth—the day He kept every week as the Sabbath. (See Luke 4:16.) The days of the week have never been confused. Here’s why some people ask this question …
Before 1582, the world went by the Julian calendar, named for the Roman Emperor Julius Caesar, who implemented it in 46 BC. The Julian calendar had calculated that it takes the earth 365-¼ days to orbit the sun. However, it actually takes about eleven minutes less than that. Those eleven minutes accumulated each year until, by 1582, the calendar was 10 days out of harmony with the solar system.
On October 4, 1582, Pope Gregory XIII issued a papal bull proposing a new calendar—the one we go by today—called the Gregorian calendar. As countries switched to the Gregorian calendar, they would “lose” a number of days, bringing their calendar into closer alignment with the solar system. The longer a country waited to switch, the more days would be dropped. Most European countries switched right away, and they lost 10 days. For instance, Thursday, October 4, 1582, was followed by Friday, October 15, 1582—not October 5. (The United States switched in 1752, making 11 days disappear. Turkey was the last country to switch, making the jump in 1927, and it lost 13 days.)
Yet no matter how many days a country lost by switching to the Gregorian calendar, the weekly cycle was not affected at all. Friday still followed Thursday, and Saturday still followed Friday. The same seventh day remained.
And because the weekly cycle is completely independent of the lunar and solar systems, we can be positive that when we rest and worship in observing the Saturday, it is the same seventh day on which the Creator rested, and on which Adam and Eve, the Israelites in the wilderness, Jesus, and the apostles, rested and worshiped.