John Locke was a political philosopher whose work, 'Two Treatises of Government' in particular, heavily influenced the creation of the United States of America, and, more specifically, the Declaration of Independence.
Without Locke's specific theories (which were influenced, in turn, by Thomas Hobbes) on the social contract, the state of nature, and natural rights, the United States today, as well as nearly every nation state in the world, would not exist as we know it without this man.
August 29th, the day Locke was born, should be given to him in recognition of the influence he has had on political, philosophical, and moral history to this point in time. Not as a federal holiday, but simply as recognition.