The free evening, the one square a working week leaves
The free evening is the one night of a working week with nothing printed against it, and on most trips a couple gets exactly one of them.
On a trip built around somebody else's programme, the evenings are not interchangeable. The first goes to arrival and whatever the company organised. The middle ones belong to the schedule and to dinners nobody chose. Somewhere in the week there is one square with nothing printed against it, and that square is what this site calls the free evening.
It has a short shelf life
A free evening is only free until somebody else notices it. By Wednesday afternoon a Thursday with nothing in it tends to have collected a client, a team dinner or a drink that cannot reasonably be refused. Couples who fix the hour early are not more organised than everyone else, they have simply learned that an unclaimed evening does not stay unclaimed.
Holding it without locking it
Fixing a date is not irreversible. One change of date is free, provided it is asked for more than twenty-four hours ahead and the new date sits inside a fortnight, which covers nearly every genuine agenda change. The practical move is to claim the square early and keep the free change in reserve.
Which square to pick
Not the arrival night, when one of you has just flown and the other has been walking since morning. Not the night before the earliest start of the week. Usually the night after the last full day of the programme, or on a longer trip the Saturday added at the end. The page on a working trip goes through the week square by square.