I know it had SOME effect, I think it gives your culture creep a big boost making it easier to flip opposing cities. Exactly how much or even how that works in Civ Rev is not clear to me.
The "I Love the King/Queen Day" give a +5 culture boost in a single city for one turn only. Usually occurs when culture is doubled in a particular city, or when acheiving a significant overall culture increase throughout your civ. You'll often see it after you get the Ark of the Covenant, or whenever you build some wonders.