Monday is the key of the week

This appears to be a musical analogy. If you hit the right note to begin with the melody of the week will flow harmoniously. Getting off to a good start sets the tone for what follows -- well begun is half done. Start off on the right foot.

Thinking of locks it might refer to opening or beginning when you prepare for what is to follow. A good plan or strategy for the week will ease the flow of work and increase productivity.

Thought of as a keystone it might be seen as the connection between one week and the next.