Deciding when to mail your save-the-date announcements can be tricky. Peggy Post says to send these three to six months before your wedding, which allows guests plenty of time to make necessary travel arrangements, book a babysitter, and save money for airfare and wedding gifts. It isn't recommended that you send your cards too early, because guests are likely to set the notice aside and forget or misplace it. Mailing them too late will practically overlap with your wedding invitations, which is a waste of your time and money.
Don't leave anyone out! You must send a save-the-date to everyone on your guest list, even if you've already shared your wedding date with some of them in person, via e-mail or over the phone. Once you send someone an announcement, you must also invite her to the wedding. Make sure your guest list is set in stone before your announcements go out, because after that, you really can't make any changes. However, if you find out later that you accidentally left out your aunt's cousin's brother and you have to invite him to the wedding, you can certainly send him a formal invitation; these guest list additions happen.
After you drop off your save-the-dates at the post office, sit back, take a deep breath and feel better that you've crossed one more item off your to-do list. You're one step closer to saying "I do!"