King Agnarr and Queen Iduna of Arendelle
perish at sea on their way to Rapunzel’s wedding (jan 1, 1836 – dec 31, 1836)
This is not canon, however, we see Rapunzel and Eugene at Elsa's coronation, thus it can be assumed they were returning the visit out of both sympathy for the loss, and royal duties (evidence has been found in many fan theories about this).
Added to timeline:
The Big Five Timeline
This is a timeline of the "Big Five" and how they line up hi...