as to your question on why the catholic church assigned the days they did, i don't know. i have heard theories, and i have my suspicions. However, for the time being, i am trying to keep with the historical facts given in the Bible, so i'd rather not go into that part of the discussion. i do have a great idea, though... perhaps you and i can go on a fast and see if God tells us. i propose we start at 11:58 PM and end it 24 hours and 4 minutes laterat 12:02 AM. then we can say we fasted 3 days and three nights!

Gal 2:16 Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified.

Jas 2:24
Ye see then how that by works a man is justified, and not by faith only.