Will be in Salzburg on April 30 and May 1 ...will sites and transportation be available or will everything be closed for May Day?
Transportation will be on holiday schedule. That means it will be available but not as frequent as on work day.
April 30 is not a holiday, just a normal Wednesday. May 1 is a national holiday, so shops will be closed and transportation will run at a reduced/holiday schedule. All else, including sites and restaurants, will operate as normal.
Nothing to add. Happy travels!