Fortunately, WoW sport designer Jeremy Feasel affirmed the zombie invasion part of the event will only be active for a couple of hours through the TBC Classic Gold day rather than all the time. I'm actually a bit disappointed with that, frankly, because I think it'd be fun to find yet another all-out, 24/7 invasion that players need to attempt to contain--something similar to World of Warcraft's notorious Corrupted Blood incident.
Players may also venture to the arctic wastes of Northrend to battle bosses and earn strong loot, and there's also a new story quest to try out.
Otherwise, you can wait until the event necessarily launches sometime in the forthcoming months. But now that it's about the PTR, it'll likely cheap Burning Crusade Classic Gold be happening sooner rather than later