The June 19, 2012 Patch added 7 new crafting or "clue" items: Goldfish, Pocket Lint, Cheese Wheel, Banana Peel, Barn Door Plank, Secret Diary, and Damaged Capacitor. These all had silly descriptions describing them as "mysterious" or otherwise unusual.
On June 20, the game was patched again, making the item set Eliminating The Impossible. When a player equipped the set, new items descriptions were shown on the clues. These new descriptions forms a quote from the Sherlock Holmes story "The Adventure of the Copper Beeches" .
Another patch on June 21 added a crafting psuedo-recipe: 2 Banana Peels crafted together resulted in Error Codes. The error messages were each part of 1 of 8 images. The images were QR codes, which further linked to other images. These 8 Hidden Images formed part of a transcript of a hearing with several US senators and Saxton Hale, regarding the Poopy Joe incident.
Adult Swim announced a new promotional event involving Valve. Coming next week, it appears to involve TF2 specifically.
The text on the side of the Damaged Capacitor indicated the date/time of June 22, at 15:00 (or 18:00, depending on taste) US Pacific time. At 15:00, a user called "The Pyro" made a post to the TF2 blog.
Mmmmph mmmmph, mmmmm mmmmmp mmmmmph. Mmmmmph mpm! Mmmmmph mph mm! Hm mmmmmph, hmmmmph mm mmmmmph mmmmmph!
xkeeper translated this to Morse Code (taking longer words as dashes and shorter ones as dots), which yields:
-- --- -. -.. .- -.--
If done with the title, It will do this:
It comes out in A or ET
In Turn, It says A MONDAY (or if you want to do it the other way, ET MONDAY)