We've had negative 20 temperatures for over a week which happens about once every 10 years so the demand is extremely high and on top of that few of our powerplants are out of service for maintenance so that electricity has to be bought from abroad too.

Few cold days in a row is not an issue as buildings still have heat stored up in the structures but when it lasts for a long time the demand for more heating goes up drastically.

I'm don't know how energy contracts work in Finland, but in Germany you usually have a fixed price per kw/h. That price may change frequently, but it has to be announced and you have the right to cancel the contract each time.

The graph OP showed looks like the price development on the spot market, that's where energy providers buy energy short-term, apart from their long-term contracts. Spot-market-energy is naturally more expensive than the long-term one. That price may also be very unstable, as for example an unexpectedly cood winter week among several regions/contries can let it hike up pretty drastically.

AFAIK, this short-term price is an option for the private consumer as well. It has the advantage of being much cheaper most of the time when demand is low/normal but the disavantage OP shows here.