From Harold Bloom's Genius

[It was Sara's birthday Monday which reminded me of this.]

"The ancient Roman made an offering to his genius [originally a person's attendant spirit] on his birthday, dedicating that day to 'the god of human nature,' as the poet Horace called each person's tutelary spirit. Our custom of birthday cake is in direct descent from that offering. We light the candles and might do well to remember what it is that we are celebrating."