A date pun for the Japanese word niku (肉) "meat". It can be on the 29th of any month, or on February 9th (2/9), as the digits 2/9 are "ni"/"ku" in Japanese.
Meat is a euphemism for "excess flesh" in Japanese, and eating red meat is likewise depicted as an easy way to get fat quickly, so pictures of curvy, plump, and fat girls start appearing on these days in greater numbers, sometimes alongside actual meat.
Alternatively, some artists consider "meat day" as a celebration of characters with meat, in other words, muscular.
