User blog comment:Bowser & Jr./The 2013 Random-ness Wiki Calendar!/@comment-4059089-20130101221402

September 23, 1889: Nintendo was founded.

So Sept. 23 could be Nintendo day! However, it wasn't until WAY later until they made games, so...

September 19, 1985: Mario Bros is released

So Sept. 19 could be Nintendo day.