Without looking up your itinerary....the NORMAL would be that the second night (not the day you board the ship but the next one) would be formal and the other one on a seven day cruise almost always is the second to last night. For the most part you are right to assume the day at sea on the return leg would be formal.....but, again I haven't checked the itinerary.....it is usually as I said above. If the day at sea is that second to last night...you can be positive that would be formal.
First and last night are always casual on HAL.
I am assuming that the Zaandam will have begun her seven day cruises out of Canaveral by then.
Marco Polo Reservations: When you board, you will find a daily program on the counter/or bed in your cabin. It will state where and when to make your alternative restaurant reservations. It fills up quickly, so if you really want to dine there...don't delay in making your reservations.