May 17, or "syttende mai" as it's known, is far and away the biggest street party Norway has all year, easily eclipsing New Year's Eve.
Norwegians pull out all the stops: Girls and women come out looking like Heidi in their colourful dirndl dresses, while the men and boys look like Georgian gentlemen in frock coats, top hats and 17th century shoes. Often, Norwegians will also sport the tartan pattern traditional to their home province.
Some of the costumes cost 70,000 kroner ($12,000), so all this dressing up is something the country takes very seriously indeed. Children's parades make their way through every town and city, to the proud cheers of parents, before everyone sidles off for an afternoon's festive drinking.
It's all rather spectacular, but as a foreigner you can feel a more than a little excluded by this manic focus on Norwegian-ness, and it's easy to suspect that recent immigrants to the country are only grudgingly invited to the party.
Erika, a Swede who's lived and worked in Oslo for more than five years, says she normally takes the chance to go home to Sweden.
"I normally 'escape' the country on 17th May," she says. "Being a swede and not that into nationalism, it's kind of weird. It's normally not that bad, and kind of cute, but on a day like this, it's too much."
But escape is not the only option.