It's "Pineapple Day", almost certainly because of another number pun, although I'm not sure how. (8 = hachi -> ha -> pa, 1 = ichi -> i, 7 = nana -> na, and somehow "puru" comes from somewhere.)
Right?It's Hya-haa!!
Hya-haa is a Japanese nickname for Jun-you, due to her catchphrase laughterHya-haa, right?It really is Hya-haa.Pineapple day.
Presumably, this has something to do with how 8/17 can be pronounced "paina", which is the first half of "pineapple" as pronounced by JapaneseAugust 17th