Finnish Ukko "Old man", in this context familiar term for "overlord, highest of the gods") or simply vakat (Vakkas), was a sacred festival celebrated in Finland in honor of the god Ukko.
The Vakkajuhlat were commonly held in May coinciding with the spring sowing.
[1] After introduction of Christianity, other religious activities were banned as idolatry, but the festival continued despite this.
The festival is first documented in 1542–1547, when peasantry in Savonia were fined for having celebrated what was referred to by the authorities as festivities to Thor (as the god Ukko was identified by the Swedes as the Finnish version of the god Thor).
[1] Reportedly, the festival was still celebrated in at least some part of the country as late as 1910.