This quest becomes available once you have completed the main story, which involves completing all the pledges to each region from the alliance map at the longhouse in Ravensthorpe. You must also have built Ravensthorpe to at least settlement level 5 - village before the quest will become available.
A few days after the story completion, Gunnar will have a quest marker above his head. Speak with him to find out what’s on his mind. He’ll tell you that he and Brigid want to get married, now that things have settled down a bit. When you’re ready, tell Gunnar that you can begin the wedding preparations immediately. A cutscene will begin which depicts the wedding, after which you’ll have an opportunity to speak with the guests, who are almost all residents of Ravensthorpe.
Once you’ve spoken with guests at the wedding, speak with Randvi, who can be found leaning against a tree to the northwest. After walking with her a while, you’ll have the opportunity to begin a relationship with her if you haven’t already, or tell her that you’re grateful to be her friend. If you do choose to begin a relationship, Sigurd will be happy for Eivor and Randvi.