Calling setup_postdata prior to wp() being run breaks the query.

[prettyboymp]

I was writing code to register sidebars based on a posts of a custom post type. While doing this, I noticed that the manage posts and manage pages screen was showing "No pages found".

After looking into, I found that there is a collision with the global variable $day. Its set as a public query_var, but at the same time is used many other places with a different purpose.

My thoughts are that its safer to change the name of the global $day than to change the query var.

[nacin]

Looks sane.

[ryan]

(In [16303]) Prevent stomping of global. Props prettyboymp. fixes #13945