Flying Standby In September, Should I Worry?

[Disneylvr]

My husband and I decided to definately use our Delta "Friends and Family" vouchers In Spetember flying from STL to MCO(connections in Atlanta). We would leave either the night of September 13 (Thursday) or anytime during the day on September 14 (Friday). We would return home on September 18 (Tuesday). I am aware that Friday is NOT a good day to fly standby but we are out of vacation time from work. As of this morning, all the Delta flights on both our arriving and departing days are wide open with LOTS (in some cases well over 100) of seats remaining in both coach and first class. If available we will be put in first class. For all you airline experts, is there still a good chance that these flights could suddenly be reserved even with only 3 1/2 weeks before departure? Are there lots of seats that aren't assigned until day of departure? Thanks.