"Der Samstag ist der letzte Tag der Woche."
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.