There's new content out, so ArenaNet wants you to play it ... then buy the game.
But the folks at ArenaNet have a heart. They want those players to still be able to enjoy the game and listen to all the high level characters talk about their fun experiences with the new content. So they are extending the free trial period through Sunday night.
And, of course, once they find out how much fun the game can be, they will want to go out and buy it. At least that's the theory any way. So ArenaNet is also extending the sale two days on the game (the Digital Heroic and Digital Deluxe editions) through Tuesday night. If interested, it will cost $39.99.
Source: Official site