DigiParty 1988 was held in Hartola, Finland by Digitize Design Group. Another DigiParty was held in 1990.
It appears there is some dispute over the date; Demoparty.net and Janeway claims april, while CSDB and the party report in Scandinavian News 1 claims may. All claim 20th to the 22nd, though. If we look at the calendar, we can see that only in may did that date correspond with a weekend, leading us to believe may is the correct date.
Party report in Scandinavian News 1 (july 1988).