This article explains how to block individual days in the price planner.
Any blocked day can't be used as either arrival or departure day.
1) Go to Properties > Edit > Price planner (in drop down menu to the right of the property) > Go
The Start days row in the price planner can show three different "states":
a) covered by short breaks
b) default arrival days for 7 night, or multiples thereof, stays
c) blocked date, guests can neither arrive nor depart

Please note:
When you have added short breaks, but need to check which date have been enabled to allow 7 night stays via the property settings, you need to delete the short break band, check the dates and then add the short break band again.

In order to block / unblock a date click into the square in the Start days row for that day.
When clicking into an empty square the
1st click enables the date
2nd click disables the date
3rd click changes the square back to the original (white) state
When clicking into a square currently set as a default 7 night arrival date the
1st click will disable the date
2nd click will enable the date again
Please note:
When the date is a default 7 night arrival date it is not possible to set it as a white (blank) square. It is only possible to enable or disable it.