I know that every kid hits milestones at different times, and there's no real "right" age, but roughly when should by son (now 4) start to really understand things like "next month"?
He kind of, sort of, knows his days of the week, but still has trouble conceptualizing "we're going to visit your friend a week from tomorrow... today is Monday. Tomorrow is Tuesday, then Wednesday, Thursday, Friday, Saturday, Sunday, Monday again, and then Tuesday again, and we'll see her then."
Trying to explain things like "how long until Christmas" seems pretty futile.
At what point should we start to see him being able to make these connections? Are there things that we can do to help him gain a better perspective on time?