I thought the German week ended on Sunday.
I thought everywhere it ended on Sunday. :)
In the US Sunday is usually considered the first day of the week.
Me too. ;)
Why "The" saturday ....is wrong?
I think in English you don't put articles before the days of the week, unless you are speaking of a specific "Saturday". (But I am no native English)
The first half of your sentence was correct. You never use "the Saturday" by itself. If you're talking about a specific saturday, you'll say "that Saturday", "last Saturday", or "the Saturday I lost my pants", whichever is applicable.