Our departure day varies - depends almost entirely on when we can get reasonable airfare out of MSP.
According to MS, the busiest check in day for DVC resorts is Sunday and the busiest check out day is Friday. That used to matter when rooms were assigned and requests granted based on when the reservation was made. But under the new "room ready" assignment policy, knowing the most popular check out days isn't going to help you alleviate your wife's concerns.
Here's why (and I'm sue your wife already knows this):
DVC resorts are nearly always at (or very close to) 100% occupancy. Rooms that aren't reserved for points by DVC members are released to CRO/WDWTC to rent out to the general public for cash.
For those paying cash, weekend nights are the same price as weeknights. December weekends are very popular times to visit. BCV will be full pretty much every night in December so it won't matter when DVC members tend to leave as there will always be someone checking in to replace those who leave.
If DVC doesn't "pre-assign" you rooms close together, you'll be at the mercy of the CM checking you in and the number of unoccupied rooms available at the time you do check in. Under the "room ready" policy, it won't matter if the guests who checked in ahead of you were staying on points or paying cash. It's pretty much first come, first served.
I'm sure zulaya is already planning to request that the rooms be close together when she reserves them. As others have suggested, I believe that some requests are pre-assigned and hope that a request to be close together is one of them. IMHO, it wouldn't hurt to keep other requests to a minimum (maybe only a smoking or not request in addition to the "close by" request) and to arrive early on check in day, too.
Best wishes -