In "Getting Things Done", as I remember it, it says not to put an action on the calendar
unless it's something that, if you're going to do it, has to be done that day.
A concert you might or might not attend fits this.
As I remember it, he said a reason not to put a bunch of actions on the calendar is that
typically you don't get all of them done, and that it's time-consuming and demoralizing
to have to keep recopying them onto another day. I used to do that, and agree with
him on that; it may feel fine for the first while but started getting me down. If some
people have more predictability or are able to put a few actions on the calendar without
overdoing it, and only occasionally have to recopy a few and don't feel overwhelmed by
that, then fine -- they can put actions on their calendar, as far as that reason goes.
If I remember right, David Allen also said you can make an appointment with yourself
and mark it on your calendar. This seems to me to contradict the other things
he said about calendars, especially in light of another thing he said, that you can
renegotiate agreements with yourself at any time.
I don't think David Allen ever said to just do things in a context without considering
importance. Within the context, he does say to consider priority as one of the factors
in choosing things. As far as I remember, he didn't say anything about how to decide
when to move from one context to another; I assume this is one of many things he
expects his readers to be smart enough to figure out without his help, or that
different people will do differently so there's no point his telling people one method;
or that can be carried out using one or another GTD method such as Roles,
Weekly Review, or Projects, etc.