Brittsommar is the Swedish word for a late period of warm weather. In English you might call it an 'Indian summer', and the term indiansommar also exists in Swedish.
Brittsommar doesn't have any link to Great Britain, but literally means 'Britt summer', after Saint Bridget of Sweden, the only Swedish saint to have been canonised. The Swedish version of Bridget is Birgitta, sometimes shortened to Britta, and she has her saint day on October 7th.
You usually use the term to describe any period of warm weather in early autumn, if it comes after a cool and more typically autumnal spell. Unlike autumn and winter, Sweden doesn't have a specific meteorological definition for a brittsommar; there's no set date or temperature requirement.
But what's it got to do with Birgitta? It's not just about the timing of her national day. According to the meteorologists at SMHI, she wasn't a fan of the cold Nordic winters and prayed for warmer weather, so extra days of sunshine around her saint day are seen as an answer to these prayers.
SMHI says that a period of distinct warm weather in early October is rare in Sweden, but has happened in 1973, 1995, 2005 and 2018.
In older Swedish, the term grävlingssommar, literally meaning 'badger summer' was used to describe a late period of warmth. This was linked to the fact that badgers are active around this time collecting supplies for the winter.
Blir det någon brittsommar i år?
Will there be an Indian summer this year?
Brittsommar på väg in över Sverige
Late summer weather on its way to Sweden