My Expression works perfectly in the desktop version. Than I put it on the server and trigger(onsheetactivate) does not work. :-( ...
Later I searched the forums for the triggers, and found some disappointing discussions. Reload and On Open trigger seem to work properly.
I could live with that, when I can manage to set a clear state, which is same like the onload. Else the user gets all weeks shown which are in system. I tried a bookmark, but I couldn't get it to work, that the bookmark is updated for the actual date. Best way would be to shrink data, but it's no sensitive data, so it is no problem if the user has the possibility to explore the data.
@Gysbert: I don't use any buttons, in fact there is only one sheet at the moment, because of the problem described here.