Starting Thursday at 12:01 a.m. and going through Sunday just before midnight, Forza 5 will be available in its entirety for free for Xbox Live Gold Members.
The full game is coming in full including achievements and all cars available in the regular purchased game. Subscribers can hop into any car of their choice and drive for the entire weekend earning achievements and raise their gamerscore. After the weekend is over, that gamerscore and achievements stay associated with the account. It's like Games With Gold for a weekend.
It's pretty safe to say that if this is popular and successful, we can probably expect promotions like this more often.