BerlinWalk tool
Berlin does not get busy evenly. It gets busy in windows, and most of them are trade fairs out at the Messe grounds in Charlottenburg. Put your arrival date in and see what your stay lands on.
These come back every year and they are not on the grid above, because the organisers have not published a 2027 date. If your trip touches one of these months, check the organiser's own site before you book.