Why is Sunday the first day on the calendar when it is considered the last day on the week in daily living?

In the Old Testament of The Bible - which is a Jewish book - God made the world in 6 days and rested on the seventh. This rest day, the last day of the week, was Saturday, and became the Jewish sabbath. Jesus was crucified on a Friday and rose again on the Sunday. Christians keep Sunday as their holy day. This does not change the fact that Sunday is the FIRST day of the week The modern habit of considering Sunday the last day of the week has arisen since the adoption of the Word WEEKEND to mean Saturday and Sunday. This does not change the fact that Sunday is the FIRST day of the week