londrum - 8:51 pm on Jun 23, 2011 (gmt 0)
i wouldn't remove them at all, because you might still get a bit of traffic out of them.
what i would do is give them a "noindex" tag after the date so google can remove them from the index, and then include a little header at the top of the page which points people to your new content.
i dont think that automatically redirecting people to new content when they are expecting the old content is a great idea. what if they are visiting the page just one day after it has "expired"? they might have visited the page a few times already and know what they want. just stick a header at the top explaining that its now out of date and point them to the new place.