I've always started my weeks with Monday and ended them with Sunday. Seems to make more sense to me. However, when I was training for my Ironman, my friend who coached me always started my training schedule on Sundays. Drove me crazy.
My weeks are so crazy. They begin and end in a different spot all the time.
LOL... just pick the one you "feel" on any given Monday! Embrace the dichotomous nature. LOL... am I of absolutely NO help whatsoever?