Our idea of a week can probably be traced back directly to early Israelite religion. There’s good evidence of a 7-day cycle by the 6th century BCE, maybe as early as the 9th. They cared which day was which because the 7th day was a holy day of rest (still practiced as Shabbat), matching up with the supposed final day of creation. Jews have been keeping this practice for a very long time, so I would guess that we’re still aligned with the days the ancient Israelites were following that they thought matched up with the days of creation. Unfortunately, the exact nature of what day they decided was which is lost to time.