The hardest thing in the world to understand is income taxes.
After spending a good part of yesterday working on my Federal income tax, I think that perhaps a national day of mourning would be an appropriate commemoration. One hundred years ago, the United States Supreme Court upheld the constitutionality of the income tax. The vote was 8-0 with Justice McReynolds abstaining. Go here to read the text of the decision. Ironically, Associate Justice Charles Evans Hughes, who would soon resign from the Court to be the Republican Standard Bearer for President in 1916, had opposed the income tax amendment as Governor of New York in 1910.