The post-defence party (in Finnish: karonkka) is also an old academic tradition. It is a party after the public examination, organised by the doctoral candidate to thank the opponent, the custos and other people who contributed to the dissertation. The opponent is the guest of honour.
There is also a protocol for organising the post-defence party, but each doctoral candidate can organise a event that suits him or her. It is by no means compulsory to organise a post-defence party - instead of a big party, it is possible, for example, to take the opponent and the custos to dinner on the day of the public examination. The post-defence party is not an official University event and each doctoral candidate is personally responsible for the costs related to their party. This is another reason why it is entirely up to doctoral candidate to decide whether to organise a party.
For more information on the customs and traditions of organising the party, see the university's Welcome to the Public Examination website. You can also ask recently graduated doctors of your research field, supervisors and other members of the academic community for tips on how to organise a karonkka.
If the public examination is held remotely, the post-defence party does not need to be organised on the evening of the public examination, but can be arranged at a later date. However, if the opponent is unable to attend the post-defence party, it is still polite to thank them during your speech. It is also recommended to arrange for the opponent to give a short speech either remotely or via a video greeting.