We have talked about when to mail out invitations on here before, but as often as I get the question I don't think it hurts to repeat it. There is a pretty easy formula that I like to follow.
Start with the date that your head count is due back to your reception site. From that date, count back two weeks. This is the date when your RSVP will be due back to you. (A two week span will allow you plenty of time to call all those guests, which for some reason, have no idea how to mail an RSVP back.) Count backwards another six weeks and this is when you mail out your invitations. If you are mailing some of your invitations out of the country count back another two weeks and mail them on that date. Remember, if you are sending out save the dates, they need to be in the mail a year to six months before your wedding date.