There are a lot of hostels that only work seasonally and shut down completely for nearly half of the year. Others, usually smaller operations, do close their doors for a week or even a month or two in the dead season in order to save costs, take vacation, or renovate their hostels.
Shutting down temporarily is not always a question of being able to afford the time off. Sometimes it's a matter of not being able to afford to stay open. The simple combination of cold winters and tall ceilings can make heating bills astronomically high. If there aren't enough guests to cover the operational costs, then it may be better to put business on hold and open back up when there are more travelers.