This sounds like it should be a simple question:
On what day was the US Declaration of Independence signed?
But, like all of our search challenges, there's more to this than meets the eye. Even though this seems dead obvious, finding the actual answer requires a bit more sleuthing... and--more to the point--a deep, abiding willingness to check out things you're just SURE you know already off the top of your head.
Since this is a holiday edition of the challenge, this won't take you long. But it will provide you something to chat about over the July 4th barbecue.
Just give us a quick answer (and, of course, say how you came to that conclusion)!