A date pun that occurs on November 29th, or 11/29, as the 11 looks like イイ, the Katakana spelling of いい (ii), which means "good, nice, pleasant", and 29 contains the digits 2-9, or as said in Japanese, "ni-ku", the word for "meat" (spelled niku, にく, or 肉).
It generally serves the same purpose as meat day, but on a grander scale, where pictures of voluptuous, plump, fat, and obese girls start appearing in larger quantities.
