Independence Day in Cambodia

I do not believe the setter expected anybody to know the answer to this question; how can we remember the independence day of every country that's ever been subject to rule by another? This is a classic of the 'at least you can have a guess' genre, but that will not do either. As I may have said before, this is not supposed to be about guessing; it's supposed to be about what you know, or about things at which you can at least make an educated guess (which is an actual thing by the way, and not just something that people on The Chase claim to have made when they've actually had to make a wild stab in the dark).