Bikini Day (Also known as every 23rd of the month) is a meme where talented user The Trav will post drawings of people in Bikinis, often drawing characters like Wario and Ganondorf and ones that are not considered "sexy." It branched off from him though, and now many different users draw for Bikini Day.
There are so many Bikini Day drawings that you can find by searching "bikini day miiverse" on Google, but you can find some at the gallery section.
Bikini Day is often met with mixed reception by Miiverse users. There are many who are fine with Bikini Day drawings, while others think they're absolutely disgusted.
More drawings are located here.