The first written evidence for it dates back to the 6th century. The Kukeri are in fact people who dress up in eccentric costumes sometimes resembling animals but on many occasions not resembling anything familiar at all. In addition, they have large copper bells attached to themselves. With their scary appearance and the loud sounds of their bells they are meant to scare away the evil spirits.
You could enjoy festivals with kukeri at various times throughout the year all around Bulgaria even though the custom is most common in Southeast and Southwest Bulgaria. Most typically, it is held either around Christmas or during Sirni Zagovezni (a holiday celebrated 7 weeks prior to Easter).
The best option for actually experiencing a kukeri mask festival is in Pernik (a city situated 30km from Sofia). It is an international festival called “Surva”. It is held every year on the last weekend in January.
Similar traditions exist in a number of other European countries such as Spain, Romania, Hungary, Italy and France, to name a few. If you are curious to learn more about these traditions you could browse through some amazing photographs taken by the French photographer Charles Fréger recently posted in The New York Times’ Lens magazine.