because it's the first night that there are more candles
on the menorah than empty spaces.
It's also the only night of Chanukah, that never, ever, ever can fall on Shabbat.
So the Chanukah candles have to work all by themselves against the darkness -
they don't get any help from the Shabbat candles.
It's traditional to have an extra-special celebration on the fifth night.
(This year, we have a one-month-old, and everyone has a cold. So, our "something special" was taking pictures.)
Chag Urim Sameach - Happy Chanukah