May 13 is a Christian holiday (Ascension Day, Kristi himmelsfärds dag) in Sweden. It is an official helgdagen or “red day” (röd dag), which means banks and many other businesses and services may be closing shop.
You might want to double check if you planned on running errands or attending class that day. Some businesses may even close on the afternoon of May 12th (the eve of a holiday is often taken off as well) – and lots of people might be skipping Friday to make it a long weekend (making Friday a klämdag – squeeze day).
Note many university libraries will be closed: you can check hours of opening here (make sure to select the right date).