Norway will be marking the day, 17th May, with an event in the town of Eidsvoll, where the constitution was signed.
"The King has decided that he, together with the Queen, will be present at the special event in Eidsvoll on the occasion of the 200th anniversary of the Norwegian constitution," the Swedish Royal Court confirmed in a statement.
A statement from the Royal Court in Stockholm on Thursday said that “further information” about the nature of the celebrations lay behind the change of heart.
A spokeswoman for King Carl XVI Gustav and Queen Silvia had announced on Monday that it was not normal practice for them to attend foreign countries’ national days and they therefore would not be attending the celebrations on 17th May.
The decision raised eyebrows, partly because of the Swedish royal family’s close connection with the anniversary, which also marks 200 years of peace between the Scandinavian countries. The constitution was notably adopted by King Carl XIV Johan of Sweden when he became king of Norway in 1815, the year after the constitution came into force.
In addition to attending the Norwegian event, Sweden's royals plan to hold a seminar on peace and friendship among the Nordic peoples at the Royal Palace in Stockholm on May 5th to be attended by the king and queen, followed by a concert at the palace chapel featuring singers and musicians from Sweden and Norway.
Olemic Thomessen, president of the Storting, Norway’s parliament, said he appreciated the change of heart:
“This means the heads of state in all three Scandinavian countries, the main players in the dramatic events of 1814, will be represented,” he said. Denmark’s Queen Margrethe has already accepted her invitation.
Respected Swedish former royal reporter Sten Hedman had been among those critical of the Swedish decision:
“The Norwegian and Danish royal houses will be coming together, but the Swedish king won’t be there because he doesn’t attend other countries’ national days. I think it’s unhistorical [of them],” he said.
History professor Ole Kristian Grimnes at Oslo University was among the critical Norwegian voices. He said the original decision was “unfortunate.”