Also just found this article online after Googling Killing Floor Summer Sideshow:
http://www.gamersdailynews.com/story-24039-Killing-Floor-Summer-SideShow-Event-Announced.html
Killing Floor Summer SideShow Event Announced
Tripwire Interactive has announced that they are kicking off a new event in Killing Floor at the end of this month and will run it until late July.
As opposed to the Christmas event, this summer event was inspired by freaky carnivals, fun fairs and amusement parks. So, the new map, the Abusement Park is pPopulated with sideshow freaks from the realms of someone’s fetid nightmare. More new achievements. A new DLC character pack - some of our favorite Perks, gone all Steampunk. And another unlockable character: our old friend Mr Foster "Steampunk’d".
The event runs from 30th June through 22nd July, so make the most of it!
Summer Sideshow Event Details
Free for everyone that owns Killing Floor – auto downloaded through Steam
All new level – Abusement Park
All creatures in game replaced with freak-shows themed monsters including Pukey the Clown and his side-kick, the Fleshclown
18 new Summer Sideshow achievements
Unlockable “Steampunk Mr Foster” playable character
Add to that a new DLC character pack: in keeping with the whole Victorian-era, freak-show theme, we thought we would go all Steampunk, with a Steampunk Character Pack. This time, our artists have taken some of their favorite perks and re-imagined them as Steampunk heroes. So now you can have the Berserker, Firebug, Medic and Sharpshooter - all in a way you’ve never seen them before!