Use of subjunctive

Question

Just wondering what triggers the subjunctive in this example sentence from the lesson:Se trata de que nos reunamos para pasar un buen ratoThanks.

Answer

Hola Marcos,I think they should have explained this in the lesson, as it is the only example using "que." I think they have used the subjunctive because "Tratarse de" is the speaker giving a subjective opinion about something, and the second clause is a dependent clause introduced by "que" and as such it takes the subjunctive. It would be good to hear other opinions. Saludos. John

Answer

Hola Marcos and John
The introductory phrase: se trata de que... is conveying a desire/an intention, so it's an expectation on how we want things to happen in the future, and as I'm sure you already know, this would trigger the subjunctive. It's very close or pretty much the same meaning as: 
La idea es que... (=se trata de que...)
La idea es que nos reunamos para pasar un buen rato. 
Se trata de que nos reunamos para pasar un buen rato.
The idea is that we get together to have a good time.
But of course, as you say, John, subjectivity could just be the most simple trigger, the same way we use it with sentences like "me gusta que nos reunamos" or "está bien que nos reunamos" etc.
Saludos cordiales
Inma