This companion actually requires the festival to be "in town", which happens on Sunday evenings starting at 9 PM (server time) and lasts roughly until 11 PM (server time).
The festivities occur on Turtle Beach in the Krasarang Wilds (northeast beach). You'll know you're in the right area and the right time if you see floating lanterns on the water just before the NPCs spawn.
The Pandaren will then come out from the jungle to the sand, set up bonfires, kegs, lanterns and set off fireworks near the giant turtle statue. At the same time, many little Wanderer's Festival Hatchlings will spawn on the beach, so be ready to battle and capture! :)