Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Hawaii to Helsinki as you normally would.
In terms of precipitation, rainfall in Helsinki ranges from 1.2 - 2.8 inches per month. August is typically the wettest month, when rainfall can reach 2.8 in. February is typically the driest time to visit Helsinki when rainfall is around 1.2 in.
If weather is an important factor for your trip to Helsinki, use this chart to help with planning. For those seeking warmer temperatures, July is the ideal time of year to visit, when temperatures reach an average of 60.8 F. Travelers looking to avoid the cold should look outside of February, when temperatures are typically at their lowest (around 21.2 F).