If you're planning a birthday party soon, you should send your invitations out far enough in advance that guests can include the celebration in their schedules. Following rules of etiquette concerning invitation timing shows your consideration, and will increase the chances of more guests showing up.
When to Send Invitations
According to traditional etiquette, birthday party invitations (or invitations for any event) should be sent to guests at least eight weeks in advance. However, many people don't plan their birthday celebrations this far in advance. It's best to send invitations as soon as possible (usually two to four weeks beforehand) so that guests will have enough time to clear their schedules and attend your party.
Try to send birthday party invitations to guests who live out of town earlier than invites to local guests. Out-of-towners will have to make travel (and possibly child care) plans to attend your event, and should be given as much advance notice as possible. For instance, if your birthday is at the end of the month, and you know during the first week of the month that you want to have a party, send the out-of-town invitations first, and give invites to local attendees the week after.
Save the Date Cards
Save the Date cards are useful for birthday parties that are planned well in advance, such as an 18th, 21st or 50th birthday celebration. If you're putting a couple months' worth of planning into the party, send Save the Date cards to friends and family once you know the date of the party so that your guests will have more than enough time to make plans to attend. A few weeks later, send the actual invitation with detailed party information, such as the time and location of the event.