Why is Japanese coming of age day on January 15th?

Actually, since the government tried to move all public holidays to Mondays to make long weekends (known as the Happy Monday system, apparently not inspired by the wild times of the Manchester band) it is now the second Monday of January. The day was founded in 1948. I’m guessing that they wanted to put it as near to New Year as they could without actually interfering with the New Year holiday, but haven’t been able to find any data on that yet.

More on Seijin no hi on the relevant page on the right