Be aware that Koningsdag is celebrated on 26 April this year: if the 27th is a Sunday, it’s moved to Saturday.
Public transport should be fine. They will run the holiday service, which might be a bit less than regular service. But especially for bigger cities you should be fine. Expect it to be crowded though.
As for flea markets: I’m not an expert, so can’t comment on that.
Be aware that Koningsdag is celebrated on 26 April this year: if the 27th is a Sunday, it’s moved to Saturday.
Public transport should be fine. They will run the holiday service, which might be a bit less than regular service. But especially for bigger cities you should be fine. Expect it to be crowded though.
As for flea markets: I’m not an expert, so can’t comment on that.
Edit: this was meant as a reply to OP.