During the weekend event, all subscribed members will receive double XP for all standard skills. In addition, Jagex is tossing a bone to free players by offering them a 1.2 multiplier for the same length of time. The bonus XP stacks with other XP boosters.
The event begins at 8:00 a.m. EDT on Saturday the 27th and continues until the same time on Monday the 29th.
[Source: Jagex press release]