You can only use the expression when you decide not to work any longer EX. We'll call it a day now and start again tomorrow morning.
It is also used by someone who is fed up, he will then ask : shall we call it a day?
The day when you stop smoking is a red-letter day (a day you will remember)
When you get retired, it is a "phew day"... (joke!)
Please, Alain, go further explaining.
Is it a way saying the meeting was outstanding, that some exceptional decisions were taken, some marking events happened at the meeting? In the sense of: We can call it a (famous)day!
Or does it mean that we can now say that the thing is over, the day is over. Examples: I've been learning years long, now I get retired, I can call it a day! I've been smoking so far, but now I can call it a day! (=I gave up smoking.) Until now I thought Luna was a catch but from now on I can call it a day.