Adam,
I was speaking to Airbnb last week, The dates that we enter for "Specific Dates" relates to the "nights" of the dates we enter, so for your situation for guests to control guests arriving on the 20th and checking out on the 22nd, you should only load, 20th to 21st (checkout on the 22nd being the night of the 21st) - I told them that this isn't very clear.
You can now also load "Specific Dates for 1 night by entering the same date for the range ie: from the 20th to the 20th will give you a one night range. So you could set it up that if some one stays on a sunday night they have to stay 2 nights (this could still be Friday or Monday)
It looks like you could also block someone checking in on a apecific date (if you are going to be away) by making a "Specific Dates" range for 1 day, with a minimum stay of 1 day (eg a Sunday) and then specifing a checkin on another day of the week (eg Monday). Thus making it impossible to checkin on the Sunday, but whatever has been set for previous days should still work eg checkin on the Saturday and out on the Monday.
Hope this helps explain what I have learnt
Cheers
Richard