I think Monday is busier in the morning, particularly at 9 am ET until the rush ends. This is a natural consequence of MS being closed on the weekend so reservations can first be made for three different departure dates every Monday, corresponding to the 11/7 months prior to Saturady, Sunday, and Monday. I'm not certain that having late hours would alleviate the rush which I assume occurs in the morning.
When I find the MS 800-number is real busy, I sometimes go to the 407 number (see the back of your membership card) and pay for the call. I have always gotten through right away (although I then wait in the queue the same as with the 800 number, but no busy signal/redial cycles needed).
Ralph