There wasn't one for Panda.
Folks complained about the zombie invasion because it had no defined end (it ended abruptly 2 weeks before Wrath launched, they never gave a timeline) and it was impossible to do much of anything on some servers. Lots of griefing (high level players getting infected and going to lowbie zones to kill folks), and in Shat (capital of the day), the zombies fell under the zones Pacifist Debuff and could not be harmed, even as they harmed you. In the end, on my server, everyone just logged off, most of my guild at the time just said "send an email when this shit is over" or "see you in Wrath". The public voted with their time and left.
Elemental Invasion was a reaction to that and sort of meh, so they just gave up for Panda and had nothing.
In any case, these "one time events" were over the course of a few weeks anyway, not a single weekend.