Indeed

Temperatures reached 20°C - albeit NOT for Stockholm City itself. It was slightly cooler in the north of the city, while warmer south of the city, reaching 20°C.

Even last year on 20 September 2024, the very LAST 20°C-day was registered. (Assuming that it will not reach 20°C in October - which is very, rare. Officially i believe Stockholm City has an absolute October record of 20.2°C on 4 Oct 1985 - but in Oct 1995 it was very close to 20°C again, yet the city itself didn't bring it to 20°C, but the outer southern suburbs did (like Rågsved).

In 2023, it was the 22 September, which got the last 20°C day. So, as you can see, it ain't anything special at all. Other than it is a little bit with sadness to day, that summer (days) are now over.

In 2022, it was the 12th September Stockholm City reached 20°C the last time. Same date goes for year 2021. Year 2020 it was the 24th September 2020.

In 2018, it was the 21th September - BUT on 13 Oct and 15 October 2018, it came very close with 19.4 and 19.0°C.

What about 20°C tomorrow ?

Well, SMHI says there is a chance for 20°C in the "Östersjölandskapen", meaning the area of Stockholm (East Svealand towards the East Coast) could in theory get 20°C tomorrow, before the coldfront swoops through in the afternoon.

I am a bit skeptic, given the amounts of clouds approaching Southern Sweden already in the evening... But, hey - who knows. Possibily maybe ?