What's the most subtle way of letting everyone at work know it's my birthday?

Happy birthday (if it's your birthday). Honestly if these people aren't your friends why do you care? Anybody that I know well enough already knows my birthday, if not then I really don't care. Work for me is a place to get business done. I'm not saying I have zero friends there, but my primary job is to do well so I can continue to make money to do things outside of work. However if this is really that important to you, I would ask the admin assistant (if your office has one) to set up a birthday notification system. This could be a manual email he/she sends out either weekly or monthly that lets people know about upcomming birthdays, and then have him/her print out that list and post it in highly visible areas. This way everyone knows whose birthday is that day/week/month. Of course I would have this be a voluntary opt in program.