Been testing some theories and may have found one at least, April 22nd was the North American release date of Diamond and Pearl. While why this would be honored is beyond me, since to my research so far no Japanese release dates have been used for the diamond dust weather effect. So it could just be a coincidence.I've decided to Google all the non-obvious dates (January 12th, February 29th, and December 31) with the wording "Japan Insert Date Here". Here are my results (Note, this is very half-assed research, so don't put too much faith in this. They may just be birthdays):
March 15th: Hōnen Matsuri
March 31st: Convention of Kanagawa
April 22nd: I got nothing.
May 1st: Nothing really.
September 2nd: Um...the formal surrender of Japan to the United States, ending World War II.
September 20th: Nothing.
October 10th: It used to be Health and Sports Day, but was changed to the second Monday in October in 2000.
October 30th: Nothing.
Yeah, I got no clue, though Hōnen Matsuri seems very likely and I wouldn't be surprised by Health and Sports Day.
EDIT: Wow, I didn't look very closely at Hōnen Matsuri's Wikipedia article. Um...NSFW... *sigh* Japan...
EDIT 2: I removed the link, just to make sure I'm not breaking any forum rules due to the article image. If you want to look it up, just copy and paste the name as I faithfully copied it.
Back to digging.