Instead of saying GW2/TERA/Rift, I think long time ago the seasonal events already ran that way.
They are just "Apple" it and made it sounds like their own invention. (but they also Apple it and made it better tho...)

So ya... it's just play like old seasonal/special events. they will go on by itself's pace, and players can join in anytime. Maybe there is a convenient button say "join public party" as in Rift.