HTML has a special input type for dates, like this: <input type="date"/>. In supporting browsers (pretty good), users will get UI for selecting a date. Super useful stuff, especially since it falls back to a usable text input. But how do you set it to a particular day?
Nice tutorial from Michael Scharnagl in which he takes a perfectly-functional comment form and progressively enhances it with very nice features. Things like custom error messaging, auto-expanding height, and even really fancy stuff like ajax and offline submission.