pineapple day

Pineapple Day is a date pun occuring on August 17, or 8/17.

In Japanese, each single-digit number has at least a few different sounds associated with it. Using these sounds, people can create phrases from a given set of numbers. In this case, 8/17 becomes "paina" (8=pa 1=i 7=na). "Paina" [パイナ] is the beginning of the word "painappuru" [パイナップル], or "pineapple".

For more of these types of days, see here.