Copyright: big dick day
November 18 is Big Dick Day, created to encourage artists to post works of chubby/fat males with large penises. The idea was proposed among the Pixiv community by user ふぇちょ巧太. Works directly acknowledging the day are indicated with a stylized 11.18.
Source
Also see
The following tags are aliased to this tag: 11/18, 11.18, 巨根デブの日, big_fat_dick_day (learn more).