For those of you uninformed in the ways of Lucifer, 666 is the Number of the Beast, as determined by the New Testament's Book of Revelations. Over time, it's come to bear an association with Satanism, thus the flight's reputation as the standard bearer of evil airplanes.
Flight 666 has been around for a while, and pilots are well aware of its humorous undertones. Last year, when Flight AY666 landed safely in HEL, pilot Juha-Pekka Keidasto told The Telegraph: "It has been quite a joke among the pilots. I'm not a superstitious man. It's only a coincidence for me."
For satanists, today's Flight 666 should come as a sad farewell, though, as it's the last flight to bear the satanic number. On October 29, the flight will be renamed Flight 954, in what is sure to be a crushing blow to Scandinavian black metal fans everywhere.