Opening hours, Gustav III's Museum of Antiquities

Gustav III's Museum of Antiquities is open only during the summer season: 

May–September*: Daily 10:00–17:00
October–April: closed for the season

May close in conjunction with official receptions of His Majesty The King.
» See upcoming closing dates external link

Holidays
May 1 – open
Ascension Day – open
June 6, Sweden's National Day – open
Whit Monday – open
Midsummer Eve – open
Midsummer Day – open

The rest of the Royal Palace is open all year round.
» Opening hours for the Royal Palace