1. You who wish to celebrate a birthday, enquire first who was born. One's true birthday is when one enters into the Eternal Being which shines forever without birth or death.
2. Of all days, on one's birthday, one should mourn one's fall (into samsara). To celebrate it as a festival is like adorning and glorifying a corpse. To seek one's Self and merge in it, is wisdom.
See The birth of our Beloved Lord.