meiling day

A date pun celebrating Hong Meiling from Touhou, celebrated on May 15. It's a pun on her Japanese name Meirin, with the 15 representing the 15th periodic element P (Phosphorus), also known as rin (磷) in Japanese.
May + 15th periodic element P rin (磷).

The same day is also used to celebrate Horikawa Raiko.