The rule stating that if you are dating someone or "talking" to that person, you cannot make them choose any plans for future engagements within 10 days of their actual birthday. UNLESS that person requests to make the plans. All other plans must be made by the person whose birthday is not within 10 days.
Dylan: Caroline, you have to make the plans for what we do tomorrow.
Caroline: No I don't you ginger, my birthday is in 8 days and its the birthday rule that everyone knows.
Social Networker connected to your social network and has access to your personal information which includes your date of birth, and uses this information to send you "Happy Birthday (your name here)" post to your cyber-wall. The Social Networker may not actually know you, but has been added to your social network previous to your birthday.