The weekend after the workshop i.e. 23.-25.6. is the annual midsummer festival Juhannus. It is a nightless night inside the polar circle (visit post workshop tours for more information on how to participate a tour in Lapland), and even in Helsinki the sun sets only for few hours.
The Hanasaari congress center closes on Friday at noon, which is the absolute latest check out time. In addition, all the stores close at 1 pm and will be closed the whole Saturday, neither is there any public transportation until noon. Most shops are open on Sunday.
The average weather in June in Helsinki looks like this. Here you can find a weather forecast for five days.
For tourist information you might like to visit the following links: