You can add them in advance or even last minute 

I prefer last minute (when you are really sure to catch thr train) 

Because if you miss the train or get ill and travel on few dates later its difficult as you cant deactivated a travelday on itself. Example. You can cancel the 15th May (selected travelday) only till 14th May 23.59. If you notice in the morning you cabt cancel the travelday anymore.

Thank you very much.