The medical name for having eyes of different colours is Heterochromia Iridis. You can go to Google and search for this condition and you will find a lot of information. I could not find the exact rate at which it occurs. Maybe one in 10,000 or 100,000. Its cause can be genetic, or from a serious injury or shock to the body either as a fetus or as a child. I could not find out anything about a National Day for Heterochromia Iridis.