I can't be exact although City Hall can probably tell you...but here is when we have seen them.
Around opening time riding the trolley up and down Main Street and singing.
About an hour before the afternoon parade singing in Main Street and working that crowd.
Funny story about them. A couple of years ago my dance teacher and her family went. The Dapper Dans were entertaining on Main Street and were tapping. They did a dance and then would point off to the next singer and then the next and then the next....they would all tap.
So the last singer gets down and he points off to my dance teachers daughter (who was probably about 15 at the time and a very good tapper) and she goes with it and taps away and then points off to her mom who is of course very good too and she finishes up the song tapping!
The crowd loved it and of course many people thought they were planted in the crowd but they weren't - it was just all coincendental and so much fun for everyone.
Good luck finding them.
Liz