Two things to consider when sending out invitations to a party. The schedules of your guests and how you're going to send the invitations.
Since you've already stated that this will be a holiday party combined with a birthday celebration, you know that you're going to be running into schedule conflicts with other holiday events. This is in addition to your guest's usual schedules. So you want to send them out 4 to 6 weeks ahead of the event.
The second consideration is how to send the invitations. Snail mail is more personal but you can also do a follow-up with one of the on-line invitation services (which are usually free). If you've requested an RSVP, the on-line services make it easy to see at a glance who has responded and what the party attendance count is. The on-line services can also be set up to send RSVP reminders for the people that never seem to RSVP (guilty) and to remind everyone that the event is in the next couple of days.