The unwritten “rule” is to begin planning about 1 year before the wedding. Depending on where you want to have it and what type of soiree you’re planning, sooner is DEFINITELY better than later. Sometimes you cannot even book a venue if it’s very popular. It’s also good to get your wedding dress in order asap. Often, the dress stores need lots of lead time to order the dress, get it delivered and have alterations done.
Mostly, I suggest getting started because you don’t want to wake up in July realizing you don’t have anything done & have the days & weeks leading up to your wedding be filled with unbearable stress.
That said, I couldn’t agree with gailcalled more. Remember that the wedding is, after all, only one day. Make sure your marriage is more important to you than that one day.