It's week 3 of the Guild Wars 2 Welcome Back event where players can unlock Living World Season 4: Long Live the Lich for free provided they own the expansion. Players simply go to the Gem Store to snag their copy along with an Experience Booster. The free episode is available from April 9th through April 15th.
Also during the same time period, developers will be on hand playing through Long Live the Lich. The event will kick off on Tuesday, April 9th at noon Pacific / 3:00 pm Eastern in the Guild Wars 2 Twitch channel.
Learn more on the Guild Wars 2 site.