Hi there, Robin!
It's probably tough to Google an answer like this, because I'm not sure there is a typical day anymore to begin a
Walt Disney World vacation. I live near the Jersey shore, and I know it's typical for rentals there to either begin or end on a Saturday or Sunday, but arrivals at Walt Disney World can happen at any time. I think it used to be that Saturday was a big day for guests arriving, and you might still be able to assume that Saturday sees more arrivals, but more and more families are taking weekend trips that begin Thursday and end on Monday, or they base it on airline prices. It really can vary.
I can give you this information. When considering booking
Advanced Dining Reservations, if you have some of the more popular restaurants on your radar, like
Be Our Guest or
'Ohana, and you are worried about guests booking them so that it impacts the earlier part of your vacation, look first at dates later in your vacation. Start at the end of your vacation for the harder to get dining reservations, which puts you on an even playing field with other guests arriving at the same time you are.
Another tip is to check often. If you don't get the reservations you want right away, check back. Guests change plans all the time, and you might just score a reservation you've been waiting for as a result.
Good luck, and happy dining, Robin!
Anna