Hello

i'm pretty sure it was the norse pagans, since the days of the week are named after the norse gods.

The exact version we have today is a modified Roman calendar. It's origins is from the Romans, but whom they got it from, I don't know. But the oldest form of this calendar that the Romans used only had 10 months. Julius and Augustus was added to the calendar in honor of these two emperor. Although they don't deserve such a gratitude.