in relation to the promotion that launched today

Without seeing the full T&C's for the promotion my guess is that you can subscribe for the promotional period but cancel ahead of the end of that period and it won't renew once that period has expired.

IF you have signed up and only want to subscribe for the 3 months then all you need to do is sign in to your Microsoft account and got to

https://account.microsoft.com/services to 'Turn off recurring billing' for your Game Pass subscription. Once you've done that you can continue using game pass until the 3 month subscription comes to an end and you will not be charged anything
further.

Happy Gaming!

• Xbox Game Pass is an Xbox One, Xbox Play Anywhere, and Xbox 360 backward compatibility game subscription for Xbox One; an active subscription is required to play games. Titles and number of games included may vary over time
  and by country, and may not be available in all countries. Subscription continues to be charged monthly at then-current price (subject to change; plus applicable taxes) unless cancelled. Cancel anytime; see www.xbox.com/accounts. DLC sold separately. Notice
  before purchase of DLC: If you purchase DLC for a game that is later removed from Game Pass, or your Game Pass subscription is terminated, you will need to purchase the game separately to continue using the DLC. Xbox Live Gold membership (sold separately)
  required for some games and online multiplayer on Xbox One. Download charges may apply as set by your ISP. Some titles may only be available for download by account holders 18 years and older. Game Pass discounts are not combinable with other offers and are
  not redeemable for cash; discount offers exclude titles within 30 days of launch; discounts based on Microsoft Store price. Requirements, features, online services, or subscription subject to change and/or retirement. Subject to the Microsoft Services Agreement
  (microsoft.com/msa). Discount offers exclude titles within 30 days of launch; discounts based on Microsoft Store price. Some discounts not available with select titles.

Hello Light,

Thanks for coming to the forums with your question about game pass. You can purchase the promotion, then

turn off the auto renew. This way it will not renew after the subscription time is up.

Thanks,

Brad

I'd suggest waiting until mid December for it. Since Revelations just got launched, they may do a promotional thing where everything Assassin's Creed related (save for the new game) is offered at a discount. The only other sale I can anticipate would be
on the launch of the first DLC pack for Revelations.