Why does Macbeth end with a gray overcast day?
Macbeth begins with thunder and lightning, bad weather upon the heath, but there is no mention of the weather when it ends. It begins thus to set the mood for the evil supernatural forces which will guide the plot. The natural place for witches to be meeting would be in a maelstrom of some sort, where the forces of nature are stirred up.
Having said that, it's important that you remember that a play is only about 30 percent the actual words in the script; the rest lies in the actors, the director, and the producer. How do the actors understand the characters they are playing? This determines their mannerisms and how they deliver their lines, which influences whether we see them as sympathetic or not. The producer determines what side effects to add to the script, and why. He or she is free to end the play with an overcast sky, because even though it is a happy day for Scotland, Malcolm, and his newly-appointed earls, it is the sad end for Macbeth, who began as an honorable man.