Every four years on February 29 -- (custom in restaurant)

from "Morning Edition" online summary -- www.npr.org/

February 29, 2012
Traditionally, in many European countries, Leap Day was considered the only day a woman could propose to a man. At the Beefeater Grill, ladies can order the special and get down on one knee. It's a 7-ounce steak branded with the words "Will You Marry Me?"

http://www.npr.org/2012/02/29/147621400/the-last-word-in-business