En su día

En su día
 

Does this have a set meaning or is it quite flexible ie. can it refer to the future and the past.

I think in the case I read it translates (loosely) to 'had already' or something like that.

Pues en su día se le anunció que allí lo esperaban cadenas y tribulaciones.

But on the other hand I'm pretty sure I've also seen it used where it could be translated as 'eventually' referring to the future.

Gracias como siempre para tus comentarios.

... en su día
... en su momento
... en su oportunidad
... a su tiempo

You are right about something having been done "in the moment it was done, an opportune moment ... or not quite"

I agree...
It can be translated as "in due course" (later on) or "in its day" or "in its time".