Traditionally the dress code for the wedding party is black tie: suits for men and cocktail/evening dresses for women.
This is just a guideline, so variations are allowed. However, Dracula costumes are definitely NOT allowed, Morten!
For the ladies with a role in the wedding (mother-of-the-bride, mother-of-the-groom, godmother/maid of honour and bridesmaids), a completely black outfit will either mean that they are in mourning, or that they do not approve of the marriage.
During the day preparations and the religious ceremony in church, some people wear the same outfit they will wear at the party afterwards, while some others prefer more comfortable and less warm clothes (day dresses for women, pants and shirt for men, for example).
Some monasteries are quite strict on the dress code of their visitors, however, since this is a wedding ceremony in a local church, there are no rules about having to cover your head, shoulders or legs during the service.