Summer weddings are right around the corner and I have gotten the question, "When do I send out my invitations?" a lot lately. Wedding invitations should be mailed six to eight weeks before your wedding date. If you do have invitations going to international destinations I recommend those are mailed out ten weeks in advance as the mail will take a bit longer to reach them but also to come back to you. And remember, if you have international guests, you should not place a United States stamp on their reply envelope. They will need to go to their local post office to get the proper local stamp. For destination weddings outside of the United States I would recommend your invitations are mailed ten weeks before the wedding date to allow for your guests to get the hotel booked and airfares purchased.
I recommend that you order your invitations about three to four months out from your wedding day. That gives you time to request a proof from your wedding invitation store or designer. Also, many calligraphers are very busy right now and their turnaround time is about one week to two weeks to get your envelopes back to you. Talk with your invitation company or designer about getting the envelopes early so that you can get them to your calligrapher with plenty of advance notice.
And don't forget, the cost for a first class stamp goes up on May 11th so make sure if you are mailing now and reply cards may come back to you after May 11th that you place the forever stamp on their reply envelopes.
A great resource that I use often for wedding invitation etiquette is Crane's Blue Book which can be purchased at your local bookstore.