Ascension Thursday is known in Sweden as Kristi Himmelsfärdsdag, which translates to Christ Heaven Flight-Day. Here, just like everywhere else, it is celebrated to commemorate Jesus' ascension into Heaven, body and soul, forty days after his resurrection. Unlike everywhere else, though, we haven't moved the celebrations to the weekend.
Celebrated 39 days after Easter, always on a Thursdsay, Kristi Himmelsfärdsdag is always celebrated between April 30th and June 3rd, which means it sometimes falls on a couple of secular feasts celebrated in Sweden during this time; Valborgsmässoafton (Walpurgis eve, April 30th) and May 1st (International Labour Day or, indeed, the Feast Day of St. Joseph the Worker).
It is celebrated as a national holiday even if it has lost many of the traditions attached to it. It used to be the day when cows were allowed out to grass for the first time of the year and since 1924 it is an important day for the sobriety movement. It is also the first day for fishing - första metaredagen - it used to be widely believed that fish wouldn't bite before this day.
During the middle ages there would be himmelsfärdsspel - plays illustrating the events, this tradition is not very commonly practiced these days, but you can still see it in some rural parts of the country. (If Medieval mystery plays are what you're looking for Visby is the place you wanna be, where they have a Medieval week every year in August.)
The Swedish celebration of this day carries on the rebirth message of Easter and many activities, even Masses, take place outdoors to fully utilise the sunlight that's returned after months of darkness and gloom.
Papolatria Maxima: Francis' statue given veneration in Brazilian cathedral - A video from the Associated Press -- Easter mass of Paul VI at the Cathedral of Saint Peter of Alcantara, in Petropolis, Brazil (tip: Messa in Latino):
52 minutes ago