Suit or traditional costume for guys (can be rented from a costume shop close to the St Pieterskerk) and appropriate and festive outfit for girls (no shoulders showing!). The same goes for your paranymphs. After the defence, it is customary to give a reception in the Academiegebouw. Leiden university catering will get in touch with you by email and mail. The head count you are giving them does not have to be accurate. 35 people is usually a good average for all of your guests to have a drink. Additionally, a dinner with your colleagues/friends/family can be organized and it is customary to invite your opposition committee. Most of us also organize a party in the evening to celebrate!