In Sweden, most holiday days are associated with a specific type of pastry or dessert, and on December 13th when Swedes celebrate Lucia Day, it's the saffron bun or lussekatt that's centre stage.

Saffron is one of the world's most expensive spices, with the cost by weight sometimes higher than that of gold.

Like many of the spices closely associated with Christmas, such as ginger, cinnamon and nutmeg, saffron isn't a native ingredient to Sweden. It's been used as a spice and medicine for thousands of years, but the first recorded mention of saffron in Sweden comes from the early 1300s, suggesting the spice first made its way here through trade carried out during the time of the Hanseatic League.

Although the spice is mostly imported, there are a few places where saffron is grown in Sweden, including in Gotland and Skåne.

According to food historian Richard Tellström at Stockholm University, the spices became to be closely associated with the holiday season simply because of their rarity. In years gone by, these spices were hard to get hold of, so they'd be used sparingly and reserved for special times of year.

“You only have spices on holidays. And since Christmas is the most important holiday we have, Christmas food is more richly seasoned with spices. As people got more money and economic resources, the spice levels increased,” he explained to the TT news agency.

In the early days, it would only be upper class households that could use spices like saffron, but they slowly spread throughout society as ordinary people imitated the traditions of the wealthy. From the 1800s onwards, many more families had access to the previously exclusive ingredients, and the saffron bun spread.

It only became a part of the advent festivities around a hundred years ago, first in eastern Sweden before spreading across the country.

These days, saffron is a firm part of the Lucia tradition, so much so that in recent years there have been peculiar new recipes such as saffron flavoured herring, liqueurs and even kebabs. But none of these experiments has yet had the staying power of the lussekatt.

Each day until Christmas Eve, The Local is looking at the story behind one surprising fact about Sweden, as agreed by our readers. Find the rest of our Advent Calendar HERE and sign up below to get an email notification when there's a new article.