On the Foxtrot website, instead of Sunday’s strip, there’s an explanation of it is not available. Bill Amend suggests that the timing of the strip was unfortunate considering the shooting in Aurora.
The inability to respond to current events presents a problem for newspaper comic strips. They really should have a mechanism for replacing suddenly controversial strips with filller material, or maybe even something that was meant to be released later. I can understand the benefits of packaging all this stuff together weeks in advance, but this is usually content for a newspaper, so the people responsible should really have the ability to reflect current events. It seems indicative of larger problems in the newspaper industry.