Winter holidays are usually different from summer trips. Some of the sights are closed, the beautiful fountains in the parks do not function and there are no boat trips on the canals. Anyway, this should not hold you from visiting, if you are planning to come in winter. There is a million of alternative activities that will make your stay unforgettable.
This is not a full list of winter activities, as there are too many. It is always possible to select something according to your preferences and interests. In St Petersburg you will never go bored!