What does We Love the King Day do?

[Jaybe]

Civ4 WLTKD is random (ie, a small probability each turn for those cities that qualify), and results in zero maintenance costs for those turns.

I seem to only notice WLTKD in industrial/modern eras, and by then most of my cities only have a maint. cost of 2 (huge/noble games).

[Shr3dZ]

tHe other day I saw my city in WLTKD even those I had 1 sick citizen... I didnt think this was possible, maybe you just need everyone happy, or maybe with 1 sick you have less of a chance... who knows~

[Yosho]

The manual says that there is a small chance of it happening if you have no unhappiness or unhealthyness in your city, and it's size 7 or bigger.

I suspect that if you have a much higher happiness/healthiness then you need, it seems to happen more often; there have been games where I've had it happening somewhere almost every turn for a while, but that's just a guess.