秋分の日(shubun no hi) - Autumn Equinox Holiday

Here's a conversation I had with my colleague from Japan last week:

Me: Why Tuesday was a public holiday?
K: I don't know.
Me: Why you don't know? You are a Japanese, right?
K: But most Japanese don't know too.

So I asked Google instead and Google had yet again never disappoint me.

for better info: http://web-japan.org/kidsweb/explore/calendar/september/shubun.html

The moral of this story is - GOOGLE knows best!