The U.S. Constitution went into effect on June 21, 1788, when New Hampshire became the required ninth state to ratify it (Rakove, Original Meanings, 1996, pp. 121-22). The first ten amendments, the Bill of Rights, won ratification on December 15, 1791.
And here's the curious thing: of the seventeen amendments added since the Bill of Rights, five of them were ratified in February. By ratification date, of course, I mean the day by which the required number of states had voted in favor of the provision.
No other month has seen as many amendments pushed through as February, but there were also two ratified in December and three in January, so apparently winter is ski season and amendment season. This is probably not a coincidence, as obviously a state's legislature must be convened to vote on amendments and they tend to like getting their sessions in early so that by November elections people have forgotten most of the reasons to vote against them. In that regard, it's noteworthy that no amendment has ever been ratified in September, October, or November.
Anyway, the "February amendments" are the Eleventh, Fifteenth, Sixteenth, Twenty-second, and Twenty-fifth. I'll not go into what each of those amendments is for, as if you want you can look it up, but one oddity is worth noting. The Fifteenth Amendment, prohibiting the denial of vote based on race, was ratified on February 3, 1870. And the Sixteenth Amendment, giving Congress power to tax our incomes, the swine, got the required number of states to vote yes on February 3, 1913--exactly forty-three years to the day the last time the Constitution was modified.
It's actually a pity the income tax amendment didn't get ratified one day earlier; no doubt comedians could develop some funny material about the groundhog being scared back into his burrow not by his shadow but by an IRS agent.