So how do you take part? You make new characters and you do things. That’s the short version. There are several different tiers of rewards, and the more achievements you earn, the higher your tier and the more rewards you earn. The choices you make with the light and dark side will also have a long-term impact on the state of the game, as well; which side is winning will affect the state of the galaxy in the fall, as well as determining whether a light-side or dark-side companion is unlocked for players. Check out the event and get ready to spend the summer fighting for one side or the other.
Source: Official Site; thanks to TheDonDude for the tip!