Different minumum stay depending on period
The easiest way to set this up is using two rates for your season.
The first one should have a minimum 2 night stay and be available for the whole season on all nights. The second one has a minumum 1 night stay and uncheck the day selectors for the days this is not available. In your case that would be Friday, Saturday and optionally Sunday.
This setup will always allow a two night stay but only allow a 1 night stay on the days of the week you have selected in the second rate.
The first one should have a minimum 2 night stay and be available for the whole season on all nights. The second one has a minumum 1 night stay and uncheck the day selectors for the days this is not available. In your case that would be Friday, Saturday and optionally Sunday.
This setup will always allow a two night stay but only allow a 1 night stay on the days of the week you have selected in the second rate.