What?! (Is what you're currently thinking). December totally exists. Next you think I'm going to tell you that Santa Clause doesn't exist either..
Also, it's August, why are we talking about December!? (Keep reading, I promise I'm not crazy).
Right now my main clients are those getting married in February/March, or Summer 2016, and my biggest piece of advise is: December doesn't exist.
If you're planning a January - March wedding, you need to think about when you'll be mailing your invitations, and when you'll be asking for an RSVP back from your guests. December is by far the busiest month of the year for most people, and if you mail out an invitation to your wedding, or ask for an RSVP back by December 15th, don't be surprised if you end up chasing 80% of your guests to see if they can make it.
If I were you (or your stationer) I'd highly recommend sending your wedding invitations out in October or early November, and asking for an RSVP back no later than December 1st. In reality, guests generally know if they can make it to your wedding or not the moment they open your invitation, and can likely send an RSVP back pretty quick. If you're getting married in April, you can follow this timeline, or put your invitations in the mail on January 1st (what a great way to start out a new year!).
Getting married next summer and want to send out a Save the Date? Same rule applies. Send it out no later than mid-November, or wait until January.