The first will hit on Friday, November 16 at noon PST, the developer has announced. You'll be able to see the event trigger in Lion's Arch, which will subsequently then trigger other events throughout Tyria. Saturday at noon, another phase kicks in. Finally on Sunday, November 18, the final phase will start at noon. It's a one-time event chain that runs multiple hours, but if you miss any of these events you can still play the additional content caused by the world event.
So far all we have to go on officially are three screenshots that were teased with the update, mostly of picturesque vistas.