Soon.
zombie invasions can be spawned by running katie hannon or virgil tarikoss tfs, the zone it happens in is random though so it could be hero, could be villain, could be praetorian
there is also a small chance one will happen when halloween salvage is turned in
both of these can happen any time of the year
Announced 10 minutes ago.
http://boards.cityofheroes.com/showthread.php?t=275133
Any word? Started on the 21st last year, I want to do some zombie stomping!