Religious Jewish people have the sabbath, ie the 7th day, from sunset on Friday to sunset on Saturday. So the week starts on Saturday evening, once the sun goes down.
Making the sabbath a Sunday was an invention of Christians, because the Crucifixion happened on a Sunday. So in the early days of Christianity, when nearly all Christians were also Jewish, they would go to the synagogue on Saturday and then do their Christian thing on Sunday. When Christianity spread via the Romans, and most Christians were not Jewish, then the Saturday sabbath got dropped, and Monday became the first day of the working week.
At the same time, the idea of the day starting with the night and ending at sunset was dropped. That's just as well, as who wants to start drinking at the start of the day?