As per the Insider’s info, I bring you… the grand WG EU surprise.
Mission starts: 24.12.2014
Mission ends: 15.1.2015
Condition: grind 50k XP for each nation (350k XP total, premium XP and doubles count AFAIK), using a tier 6+ tank
Reward: T-34-85M tier 6 premium Soviet medium tank.
Each nation (each 50k) is handled by a separate mission. For fulfilling each nation mission, you get 3 days of premium account. After they all are completed, you get the tank.
Edit: According to Evilly, RU server will have another tank and with different dates, so it’s completely possible this mission is EU-specific.