...... Monday is the first working day for most people I believe ......
And Monday was the first (working) day for of God too, manufacturing Earth and us, according to Jewish an our Cristian myths 
On calendar they say about the end but they don't consider the beginning
And the beginning is Monday, on the myth, isn't it?
Yaey, yaey

Not many theologians would agree with that.
I don't want to turn this into a discussion about religion, but as the Jewish
Shabbat falls on Saturday the manufacturing process must traditionally have started on the preceding Sunday....
And actually the original Christian resting day was also Saturday (as it still is for Seventh-day Adventists). The French name for Saturday reminds us of this:
samedi is a contraction of
sabbati dies (literally "the Day of the Sabbath"). And according to Wikipedia "All Slavic languages derive their name for Saturday from the name for Sabbath".
Sunday became a more special day for most Christians because it was the day of the resurrection. Again the French name for the day tells the same story:
dimanche is a contraction of
dies Dominica, ("the Day of the Lord").