Talk to the Lunar Festival Harbinger in the entrance to the Undercity. You can also talk to Lunar Festival Harbingers in other capital cities.
It's that time of the year, <name>! The Cenarion Circle invites all denizens of Azeroth to partake in our celebration of the Lunar Festival. You will find the Lunar Festival Harbinger in this very city at a location marked by a blessed circle of moonlight. She can tell you more about the holiday.