Before DVD did the shift of points five years ago that significantly lowered weekend points (Fri's and Sat's dropped on average about 30%) and raised weekday, the most popular start day by far was Sunday because members could get five consecutive nights at the lower point cost. Sunday may still be the most popular start day for the same reason but it definitely has less emphasus now than it did then. Friday and Saturday are often open somewhat beyond Sun through Thurs nights, but that does not mean they are necessarily better start dates particularly when reserving 7 months out because the fact that your Friday or Saturday start date may be open does not mean the Sun to Thurs after that Fri or Sat is going to be open.
Probably more important than whether any partticular day of the week is a more popular start date is time of year you want to go. For example, Wednesday is probably a good start date for finding things if it is January 2, while Wednesday is one of the worst start dates if it is the middle of Easter week. Higher point Friday before Presidents' week is a good start date if you want through that week but it is an impossible start date at 7 months out if it is marathon weekend in Jan, Wine & Dine weekend in November, or the Princess half marathon weekend in February. It is the time of year that most effects what you can get at 7 month out regardless of start day..