12:01 AM PDT on Friday, April 19, 2013 and ends at 11:59 PM PDT on Sunday, April 21, 2013
more info can be found here
https://www.guildwars2.com/en/news/try-guild-wars-2-for-free-this-weekend-april-19-21/
I suggest you pre-download ahead of time as it is a pretty big game.