Right now you can only set one departure day per season. There is a default arrival/departure day setting in the property rules, and then each season also has an arrival/departure day setting that can override the property default.
If you were to create a "Easter" specific dates season from 26 March 2018 to 1 April 2018, you could set departure date of Sunday for that season. Then any booking that is part of that season will require to depart on a Sunday.
That gets you most of the way there, but it'd still reject bookings for that Fri-Mon (as the departure of Sunday wouldn't be fulfilled). However, you could handle that with a second season with minimum number of nights. So:
"Easter (Sunday departure)" season from 26 March 2018 to 28 March 2018 with a departure date of Sunday. That will require anybody arriving Mon-Wed and require them to check out Sunday.
"Easter (4 day min)" season from 29 March 2018 to 30 March 2018 with a 4 day minimum number of nights. That will require anybody arriving Thu or Fri to stay at least 4 nights, pushing them through until Sun or Mon.
There is one little hole there, in that anybody arriving Mon-Wed is required to check out Sunday (not the next Monday or afterward), but in that case you could override the rules and create the quote anyway.
It's a bit complex, but should work -- and definitely use the rate tester to make sure it does what you expect.
Will that work for you?
