Night Works, Too
Take a Picture of the Sky Day is not as odd as it might seem. Many people have taken pictures of the sky for various reasons - some without knowing they did so.
The main reason for taking a picture of the sky is that, later when you look at the shot (or after you develop the film, if you're old school or a professional who prefers film), you might see something you didn't notice when you were taking the shot.
That's how most UFOs are discovered. Think of the fame!
You can also take a picture of the night sky. In fact, that could be more interesting than the daytime sky. If you know what you're doing, you'll at least get a few stars to look at.
Unfortunately, if the sky (day or night) is cloud-covered, you will have to postpone celebrating this day. I think it's okay to push it off until the next clear or partly cloudy day. Some would argue that you have to wait until November 2 of next year.
I'm not that picky.
You could even argue that taking pictures of the clouds fulfills the goal of this day, but I don't think that's the intent. You really should either have some blue by day or a starry, starry night.