Wargaming began its staggered deployment for World of Tanks 8.6 update earlier this month with the initial launch in Europe on June 19th followed by deployment in Southeast Asia on June 20th. This week its North Americas turn. The update deployed to servers yesterday to bring changes to the in-game economy, new artillery units, SPG units up to Tier X, the A33 Excelsior Premium British tank, and the Korea-based Sacred Valley battle arena.
South Korea players will have to wait a little bit longer. The World of Tanks 8.6 update is scheduled for deployment in that region on Friday, July 5th, 2013.
You can read the 8.6 patch notes for a full rundown of the latest changes.
Source: World of Tanks 8.6