The Talmud1 asks the same question, and answers that the source is the verse: "And it was evening and it was morning, one day."2 From there the Talmud infers that the day follows the night.
See also Why do Jewish holidays begin at nightfall?
Yours truly,
Rabbi Menachem Posner


Jerome's comment about a "pagan calendar" Pagans worked on all days of the week. For thousands of years, Pagans considered Jews lazy because they did not work every day. The Christian sabbath is on Sunday because it was the day of the discovery that Jesus was no longer in his tomb.