Pineapple Day is 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