Up Helly Aa refers to any of a variety of fire festivals held in Shetland annually in the middle of winter to mark the end of the yule season. The festival involves a procession of up to a thousand guizers in Lerwick and considerably lower numbers in the more rural festivals, formed into squads who march through the town or village in a variety of themed costumes.

The first celebration took place in 1878, when instead of burning the usual tar-barrel through the streets of Lerwick (which caused trouble and vandalism) on Auld New Years Eve, a Shetland yoal (traditional boat) decorated with a dragon's head and tail was burned. It did not become a regular event until 1889.

There is a main guizer who is dubbed the "Jarl". There is a committee which you must be part of for fifteen years before you can be a jarl, and only one person is elected to this committee each year.

The procession culminates in the torches being thrown into a replica Viking longship or galley. The event happens all over towns in Shetland, but it is only the Lerwick galley which is not sent seaward. Everywhere else, the galley is sent seabound, in an echo of actual Viking sea burials.

After the procession, the squads visit local halls (including schools, sports facilities and hotels), where private parties are held. At each hall, each squad performs its act, which may be a send-up of a popular TV show or film, a skit on local events, or singing or dancing, usually in flamboyant costume.