Talk to the Lunar Festival Harbinger in the Valley of Wisdom in Orgrimmar. 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.