Two years ago, we went a month before my daughter's fifth birthday. It was her birthday trip, just not ON her birthday. We got a button for her, but only had her wear it the day of her special birthday breakfast. (Not that there's anything wrong with wearing it more.)
This year my son's birthday trip will be a month after his birthday, and we'll probably do the same thing.
I figure it's like birthday parties: they're usually on a weekend before or after the birthday, not necessarily on the ACTUAL day, but it still counts.