For 24 hours, on the last Tuesday of January, the town of Lerwick goes more than a little mad. "There will be no postponement for weather". That's a defiant boast by Shetland's biggest fire festival, considering it's held in mid-winter on the same latitude as southern Greenland. But it's true: gales, sleet and snow have never yet stopped the Up Helly Aa guizers of Lerwick from burning their Viking galley - and then dancing the dawn away. Amazing Blaze Up Helly Aa is a lot more than a sub-arctic bonfire and booze-up. It's a superb spectacle, a celebration of Shetland history, and a triumphant demonstration of the islanders' skills and spirit. This northern Mardi Gras lasts just one day (and night). But it takes several thousand people 364 days to organise. Much of the preparation is in strictest secrecy. The biggest secret of all is what the head of the festival, the 'Guizer Jarl', will wear and which character from the Norse Sagas he'll represent.
The Jarl will have been planning (and saving up for) the longest day of his life for 12 years or more, before he dons his raven-winged helmet, grabs axe and shield, and embarks on a 24-hour sleepless marathon.
On the evening of Up Helly Aa Day, over 800 heavily-disguised men (no women, thank you, we're vikings!) form ranks in the darkened streets. They shoulder stout fencing posts, topped with paraffin-soaked sacking.
On the stroke of 7.30pm, a signal rocket bursts over the Town Hall. The torches are lit, the band strikes up and the amazing, blazing procession begins, snaking half a mile astern of the Guizer Jarl, standing proudly at the helm of his doomed replica longship, or 'galley'.
It takes half an hour for the Jarl's squad of burly Vikings to drag him to the burning site, through a crowd of four or five thousand spectators.
The guizers circle the dragon ship in a slow-motion Catherine Wheel of fire. Another rocket explodes overhead. The Jarl leaves his ship, to a crescendo of cheers. A bugle call sounds, and then the torches are hurled into the galley.
As the inferno destroys four months' of painstaking work by the galley builders, the crowd sings 'The Norseman's Home' - a stirring requiem that can brings tear to the eyes of the hardiest Viking.
Tears of mirth are more likely as the night rolls on and more than 40 squads of guizers visit a dozen halls in rotation. They're all invited guests at what are still private parties - apart from a couple of halls where tickets are on sale to the general public.
At every hall each squad performs its 'act', perhaps a skit on local events, a dance display in spectacular costume, or a topical send-up of a popular TV show or pop group.
Every guizer has a duty (as the 'Up Helly Aa Song' says) to dance with at least one of the ladies in the hall, before taking yet another dram, soaked up with vast quantities of mutton soup and bannocks.
It's a fast and furious night - and a lucky guizer who arrives home with a completely clear head before 8.30am the next morning which, not surprisingly, is a public holiday. Lerwick's a ghost town but by evening the hardier merrymakers are out dancing again, this time at the 'Guizer's Hop'.
That's not the end of it, for throughout the rest of the winter each gang of guizers will hold their own 'squad dances' for family and friends. By early autumn, there'll be the first meetings to arrange the next year's performance, while at the Galley Shed in St Sunniva Street the shipwrights, carpenters and their helpers will be starting work on the new galley, not forgetting 'the boys who made the torches'.
'From grand old Viking centuries, Up Helly Aa has come...' That's what the guizers sing but in fact the festival is only just over 100 years old in its present, highly organised form. In the 19th century Up Helly Aa was often riotous. Special constables were called in to curb trigger-happy drunks firing guns in the air - and dragging a blazing tar barrel through the streets, sometimes leaving it on the doorstep of the year's least popular worthy burgher. Today's festival is much better behaved.
The ingredients in the Up Helly Aa recipe go back 12 centuries and more - fire, feasting, fancy dress and, above all, fun. The torchlit procession and galley burning echo pagan Norse rituals at the cremation of great chieftains, and religious ceremonies to mark the Sun's return after the winter solstice.
Elaborate disguise was part of prehistoric fertility rites. Mediaeval Shetland guizers were called 'skeklers' and wore costumes of straw. The feasting and dancing continue saga traditions from the winter drinking halls of Viking warriors, while the satirical 'Bill' or proclamation, lampooning local worthies and fixed to the Lerwick Market Cross on Up Helly Aa morning, has precedents in the sharp wit of the Norse skalds.
If you should miss the Lerwick Up Helly Aa (or if it gives you the taste for more of the same), don't despair - there are another eight fire festivals in various districts of Shetland during the late winter.
And the country Up Hellies A' do NOT ban women from being torch-bearers and guizers. Don't mention that in Lerwick, though - where the men-only rule is a ticklish topic in these politically correct days.
The Up Helly Aa Exhibition in the Galley Shed, St Sunniva Street, Lerwick, welcomes visitors. Shetland Museum also has extensive photographic archives of the festival.