Event runs from Friday October 28th @ 10AM PDT, to Tuesday November 1st @ 10AM PDT.
https://www.youtube.com/watch?v=a1qK07YkK4k
More details here:
http://steamcommunity.com/games/305620/announcements/detail/527312139203739057
>> Event Now Live!!!
Source: Steam Community (opens in a new tab)