From Wikipedia: "The height of Kaivopuisto's popularity is on Vappu day (1 May), immediately after the preceding Walpurgis Night celebrations in the city centre. On Vappu, Kaivopuisto is packed with tens of thousands of Helsinkians, who come to have a picnic with their friends and families. Fanciful costumes, loud music, and excessive consumption of alcoholic beverages is the routine for a Vappu picnic. It is customary for all attendants who have graduated from the Finnish matriculation exam to wear their student caps at the picnic. A yellowed student cap is taken as a sign of prestige." - Hmm...what else can I say.