Dear all,
If you plan your visit to St Petersburg in mid of May you are lucky. There will be organised a very big city event "The museums night" - during the night May 17-18. Many museums will be working during the whole night. Using one ticket (350 rub) you can choose the best museums among more than 90. Their web site is www.artnight.ru (English is available with the help of Google translator only).
As far as I know, in Moscow they will have the same date.
Have a pleasant time!
Renata