Source: http://worldoftanks.asia/en/news/49/2-exp-all-victories-secret-special/

Tank Commanders,

We have received reports that our Combat Missions: Extreme has been very well received. Many of you have begun to work towards completing said missions, and the chance to receive the IS-6 Heavy Tank as a reward.

However, the going has been tough, and we would like to give you a boost. Because of this, we have decided to give you a boost with a secret special: for this weekend only, ALL of your victories in the battlefield will gain 2x EXP. Furthermore, this EXP bonus will be counted towards the mission requirements needed to get the IS-6 reward!