The best time to visit would be summer months, i.e. just about now. Everyone else has the same idea though, so I going slightly out of season, such as late April/May, or September would be good too. Great majority of hotels and rental apartments are located at the city centre, and there's little reason to stay somewhere else. Whether you prefer a hotel or an apartment is up to you; both options are available. Visiting St Petersburg (as presumably British) is not quite as easy, indeed you need a Russian visa except if you come by ferry (cruise).Edited: 20 August 2017, 02:32
Thanks weslshtraveller. show much would a ferry to Helsinki cost and would we need to stay overnight in Helsinki? Is Helsinki expensive?Sorry for all the questions!!!
Weather isn't very predictable, but most of the time in May or September it would be light jacket weather, 10-20C. Usually a bit warmer in September than in May.Edited: 20 August 2017, 02:58
The weather is always changeable with a mixture of weather, May & Sept is fine if you want to avoid the winter cold. If you look at the current weather forecast you can see a mixture of weather, I'm hoping the weather will be ok when I visit at the start of next month.
Hope this helps.
Thanks weslshtraveller most useful
May and September have the advantage of being less popular with the cruise crowd. In the summer months the Old Town is flooded with them during the middle of the day.