You’ll be able to join the celebrations tomorrow, April 22, on Embark Beach. Check out Ancient Anniversary Weaponsmith’s inventory which should contain the Anniversary set. You can also grab the Proof of Triumph which will have the knock on effect of having an extra random group of enemies and boss to appear in four late-game maps,
“…fight your way to them, and be sure to bring a Signet of Capture along to acquire a new PvE-only elite skill for each profession!”
If you miss the event, not to worry. The Proof of Triumph will work after celebrations are over. Additionally, the team will host a live stream on April 28 at 12p PDT to discuss their fondest memories over on the official Twitch channel. Finalliy, you can grab a free gift during the event from the Black Lion Trading Company.