Well, maybe in the western world from where that saying originated, but not here.
May 5th is Children's Day in the Land of the Morning Calm, turning it into the Land of Whole Day Chaos quite adequately. It's a national holiday. Kids and their 'don't spend enough quality time with my children' guilt-ridden parents will be spilling into all space deemed public in lemming proportions; if you can't abide large crowds, noisy demanding kids, cramped public transportation, epic traffic jams, and/or all of the above, I suggest either lounging and rolling at home or hitting drinking spots (i.e. non kid-friendly places) at the earliest time possible.
For those who crave mayhem and chaos and thrive from sucking the energy out of those around you (I'm talking about you crowdchasers), and those who are actually parents and have to go through this ordeal, there are many events and goings on taking place throughout the city.
All the national parks, palaces and museums have special programs which cater to both kids and adults and other festivities are being held by various districts. The zoos and amusement parks have goings on as well, of course.
Too much info to put into this post, but you can access the websites of the national museums and other parks through the Wikipage.
Additional info : the official Children's Week logo is a symbol of 5 and 5 (May 5th) as young buds walking hand in hand. Designed by Prof. Yoon Ho Sup of Kookmin University.
P.S. My can't-edit-personal-photos-yet compy problems are causing my blog to turn into a PSA! Need to get out of this mode. I feel like a virtual tour guide.
P.P.S. Caffeinated Traveller has written about the same holiday on her blog, with a more informative and much less cynical view.