Bereshit begins with the first 6 days and the Shabbat. The assumption is then that after that day, Mankind kept track of the day of the week all by themselves until today.
Are we sure that no one made a mistake between creation and today? Skipped a day? Are we sure that today is Sunday and not really Monday?
There have been generations that did not care about Shabbat (until the Hebrews I guess), and maybe they didn't care about weekdays? Maybe Noah on the Ark was not always able to keep track by himself? Or just in general - isn't a mistake possible?