Well, Lott thinks we’re all gonna die in August so I don’t think I’d be traveling there anyway.

Yeah, it would be really annoying to have all that dust in your eyes as you were asphyxiating.

They did a final cabin leak check and found a leaky cabin pressure relief valve in the orbiter cabin. Unfortunately, both valves are located behind the space toilet area back wall, which meant extra time to remove fixtures and panels to gain access (total R&R was estimated at around 8 hours).

During a meeting the next day, they decided to remove a known good valve from Atlantis (which just returned from STS-117) as a replacement. That valve was removed, then bench-tested to again verify it was working properly, then transported to the pad.

At the same time, once Endeavour’s valve was able to be accessed it was isolated and tested – and verified that it was the actual cause of the pressure leak. (It was removed, bench tested again to double check the leakage, then express-shipped to the vendor to determine the cause and be repaired).

The replacement valve was then installed into Endeavour late Thursday night and tested ok by itself. They then scheduled an early Friday morning cabin leak check which went excellent (no leakage at all).

However, since an all-up leak test is still required (cabin + airlock + tunnel + Spacehab), and since it does take extra time to set up and conflicts with lots of other internal cabin operation and setup schedules, the decision was made to add an extra day.
That way the other final pre-launch things to do in the cabin won’t be pressured time-wise by the need for a final all-out leak test.

(references and some additional details:
http://www.nasaspaceflight.com/content/cat.asp?cid=6 )