Hi Im pretty new to GTD but having read the book recently it seems to me that the logical place to put this would be in your tickler file, potentially at the start of the month.
You see it in there and ask yourself can i make use of this yet? am I going anywhere this month? Yes... ok Add it to the appropriate list to book tickets with, No... Move it forward to the next month or even to the middle of the current month so you will review it at that stage.
I would not put it on a calendar, as the calendar should be used (if I understand the book correctly) to track things that you have to do on that day/time. The example of your airline ticket is something you would like to use and not forget about and putting it on your calendar would be blurring the edges a little.
Just my thoughts, hope it helps