Depending on when you plan to travel, where you want to stay, and the type of room you want, it can sometimes be a challenge to get exactly what you're looking for. For example, if you want a concierge room at AKV, they're very hard to get because there are so few of them. Getting a Boardwalk view at BWV can also be a challenge for the same reason. It's especially difficult to get rooms at BCV or BWV during the Food & Wine festival because Epcot is so popular at that time. Wilderness Lodge is tough to book when decorated for Christmas. Any resort is tough to book during Spring Break or Christmas week.
If you're booking at your home resort and call right at the 11-month point, you have a pretty good chance of getting what you want.
If you're booking at a non-home resort and booking at the 7-mont point, the chances are lower.
If you're booking closer to the travel date, it could be a challenge to find the resort or room type you want.
If the exact reservation you want isn't available, you can put yourself on a waitlist to have the booking made automatically if someone else cancels. Members may have two active waitlists in any given use year. So, for example, if you wanted a two-bedroom at Bay Lake Tower, but the only two bedrooms available for the dates you wanted were at Saratoga Springs, you could book the SSR room, and place yourself on the waitlist for a room at BLT. Because there are three different booking categories at BLT, you'd have to pick the two you want for your two maximum waitlists. So, you would perhaps say you wanted a two-bedroom with a Theme Park View as one waitlist, and a two-bedroom with a Bay Lake View as the other waitlist. If someone cancels a two-bedroom at BLT with one of those views for the dates you requested (and you're the next one on the waitlist), your SSR reservation will be cancelled and your booking will be changed to BLT.
As to your question about the single day, in the example above, say the waitlist was for Monday through Saturday. In order for your waitlist to be fulfilled, a room would have to be available for each of those five nights. If someone cancelled a reservation for Monday through Friday and Saturday was still unavailable, your waitlist wouldn't match.
Prior to some rule changes in the past few years it was possible to have many active waitlists, instead of just two. In those days, you could also tell them you wanted to piece-meal your waitlist or all-or-nothing it. With a piece-meal waitlist each day could be booked individually. If someone cancelled any one of the nights you wanted, it would be booked for you. The downside of this was if you had a night in the middle of your stay that never came through, you wound up having to change resorts for that one night.
DVC no longer allows you to do this. You can't waitlist two separate consecutive nights on different waitlists, and all waitlists are all-or-nothing. The two waitlist limit is also very restrictive.
As a new member, you need to remember one strategy to ensure you always get to vacation when you want. Always book a room at your home resort at the 11-month point. When you reach your 7-month point, if you want to try a non-home resort, give it a shot then. If nothing is available (and a waitlist doesn't come through), you at least still have your original booking at your home resort and can still take your trip.
That leads to the second very important DVC tip: buy where you want to stay most often. The 11-month booking window is especially important if you want to travel at popular times to popular places.