Tacitus was a Roman Historian. In Book V he documented that the Jews of the time of Titus Ceaser (79-81 AD) were keeping Saturn's Day (Saturday) as their rest day, the Sabbath. This timeframe being just a few decades after Yeshua and the apostles and just a few years after the destruction of the Temple. The Julian calendar at this time only had 7 days in a week, and when we moved to the Gregorian used widely today, that weekly cycle did not change.

https://penelope.uchicago.edu/....Thayer/E/Roman/Texts