When you log into World of Tanks each day, the Advent Calendar pops up with the daily mission and offer. If you’d like to revisit the offer, or simply forgot to activate the daily mission, here's how you can access the Advent Calendar in-game:
In the Garage, click the “Store” tab at the top of the screen
From the Store, the "Specials" section should be selected by default; click it if not
Hover over the Advent Calendar window, then click “Go to Calendar”
The Advent Calendar should open, letting you activate the mission or revisit the offer.