I found at https://www.hsl.fi/en/tickets-and-fares that Helsinki area offers tickets valid for several days, from 1 to 7. Unfortunately I cannot find information if those are valid for that amount of time or for that amount of calendar days (so, for example, if I buy a 1 day ticket on Friday evening, can I use it until Friday midnight or is it still valid on Saturday afternoon)?
1-day ticket is valid for 24 hours from the first use, 2-day ticket 48 hours, etc.
The ticket is valid from the moment you show it to a card reader at the beginning of your first journey. Day tickets are valid also during the night fare hours.