Wedding is a very rare random event that can only be triggered by one of the friendly tribes on the local (small) map. You'll need to spend 50-100 food in a couple of turns to successfully finish the event and get the achievement.
Keep as many tribes unconquered as possible, keep good relationship with all the neighbours and wait. Don't forget to always keep some food to be able to fulfil event requirements. I personally got the event at turn 96.