God created the universe in six days. On the seventh day, he rested. This was shabbat, ie. Saturday. Therefore, the first day of the week is Sunday. But then

God created the universe in six days. On the seventh day, he rested. This was shabbat, ie. Saturday. Therefore, the first day of the week is Sunday. But then two millenia ago, the Son of God came to see us, and we killed Him on Friday (to be done just before the shabbat, since days started at sunset then-and-there), and He rescucitated on the third day, ie. on Sunday, so Chistians celebrate on Sunday, which became a rest day, and was therefore moved to the end of the week, making Monday the first day. Notice that Muslims have their rest day on Friday, so their weeks begin on Saturday.