It's another main quest, received after the Evil's Soft First Touches, together with Open Sesame!. Go to Oxenfurt. Shani has her own house there (M1,2) in which you will meet her and a group of officers - however, they won't cause any trouble to you. After a short conversation you will receive a task of meeting the girl near the crypt.
In the further part of the quest you will enter the crypt while Shani will wait outside. There you will have to defeat two pairs of wraiths (level 33). After you're done with them, search for the coffin of Olgierd's brother - it's the one right from the entrance, a saber is hanging in front of it.
Use the received incense by walking with it to all the places marked on the minimap. Then light the candle in the middle of the room. However, before you do it, prepare for battle. Before you will be able to talk with the summoned ghost, first you must defeat its family - five ghosts (level 33) which act like humans (and they have weapons, you can parry their attacks). Only after defeating them you will be able to get to the main quest's objective.
Before you leave the crypt, you can explore the secret passage behind the wall, uncovered with Aard sign. There you will find a corpse that has a key to the crypt. The crypt can be reached from the main corridor or by walking through the cave. However, in the cave you will find four Arachnomorfs (level 33). In the crypt you will find valuable jewelry.
Go to the wedding. Most of it is a cutscene. Once it ends, you will have to perform various activities. Some of them, like boot fishing, playing Gwent (you can't win any cards) or fight with peasants doesn't have to end with success, but they can be rewarded with large amounts of experience - and it is the sole reward. For completing all available activities you will unlock the Let the Good Times Roll! achievement/trophy. The list of all available activities is as follows:
- Drink with the girls near the entrance,
- Go with Shani to the first floor of the stable and sit with her there,
- Talk with the girl sitting under the tree in the corner of the area,
- Fish Shani's boot from the pond,
- Chase the pig (use Axii sign),
- Play Gwent with halflings,
- Dance alone in the stable,
- Drink with dwarves in the stable,
- Find the fire eater - talk with the parents of the bride.
It is also recommended to walk to the first floor of the stable. Not only to sit with Shani as part of wedding activities, but also in order to find a unique Gwent card - a Cow.