The seventeenth of May is just another day to most people, but in Oslo (and all across Norway for that matter) it’s an eagerly anticipated annual event: Norwegian National Day. A celebration of the signing of the Norwegian Constitution, National Day is a joyous and rather rambunctious affair. It has the usual parades, bands, street parties and food stalls you’d expect, plus a healthy dose of patriotic singing and flag waving. Children are allowed as much ice cream as they can ingest, and Oslo’s half a million inhabitants come out in their droves. But the twist in Norway is all in the togs.

Walk out of your door on the big day and you’ll feel as if you’ve accidentally stumbled onto the set of a historical costume drama, with everyone dressed head to toe in traditional dress. Women bustle about in floor-length woollen dresses in vibrant reds, greens, blues and purples, their laced-up bodices adorned with intricate embroidery. Little boys run around in plus fours and woollen waistcoats to match their fathers while teenagers, depending on their year in school, wear traditional fishermen’s overalls in fire-engine red and peacock blue. The effect is disconcerting at first and then, frankly, wonderful as everyone takes part and the city is completely transformed.

Don’t worry if you’ve not got the gear, and certainly don’t try to buy an outfit for the occasion as they cost hundreds (if not thousands) of euros and are passed down in Norwegian families from generation to generation. Just steer clear of jeans and wear something nice and you’ll blend right in. The best advice is to go with the flow: clap along with the packs of teenagers chanting traditional Norwegian songs; smile at the children strutting by, their faces scrubbed clean and hair done perfectly for the occasion; bow and nod to the waved greetings of the royal family from the balcony of the palace; and above all let yourself be dragged into the spontaneous and joyous revelry all around.

Oslo’s main tourist office is in the centre, behind the Rådhus at Fridtjof Nansens plass 5 (