Sending out invitations to your friends about three weeks before your planned birthday party is good enough. But you can always send the invites a month before. Remember that at this age, you and your friends most probably have your own families, your own jobs, your own plans and schedules. We already know the value of our time and some of us may even be very protective of it. So giving your friends enough time to adjust their calendars for your party and get their schedules in order means that you respect your time. However, if you plan on going on a trip with friends for your birthday, you should tell them much earlier. For example, if you want to go on a Caribbean cruise with them, then you better tell them about it months before and have them involved in the planning. They may need to file a leave from work, arrange for babysitters or dogsitters, or even save up for it.