This is a pun similar to "May the Fourth be with you". いい肉の日 (pronounced "ii niku no hi") translates directly to "Good Meat Day" - 11/29. 11 looks like "ii", and the digits 2 and 9 are prounouced "ni" and "ku". いい (ii) means "good" and 肉 (にく - niku) means "meat". Thus: good meat day.