Enabling this option will count Block dates (Holidays), Working days, Cut-off Time in Minimum delivery interval days.
For example: You have set Minimum delivery interval days: 2 (26th,27th) but from those days (26th) fall under holiday then you will have 3 interval days.
Without Include holiday in interval option
With Include holiday in interval option
In addition to this if you have a non-working day from Available Days (Wednesday) falls under interval days then you will have a total of 4 interval days from the current day.
If you have a cut-off time to 11:00 AM so it will count total 5 interval days.