In terms of your calendar question, there have been so many calendars over the millenia and I don't know if anyone is qualified to say which is "actually" right. So I will answer a slightly different question to the one you asked.

In Judaism, the Sabbath starts on Friday after sunset and continues until sunset on Saturday. This is considered the seventh and final day of the week, because this was the day that God rested from his creative acts, according to the book of Genesis.
Christians rest on the Lord's Day on the first day of the week (Sunday). This commemorates the resurrection of Jesus, marking a new era and a new creative act by God. Christians often still recognize the Sabbath as being the seventh day of the week; however most do not do anything special that day.
Muslims celebrate it on the sixth day, the day of the creation of man, from sunset on Thursday to sunset on Friday.