You can activate your pass whenever you like, but the recommendation is to wait until that you are absolutely sure of your first travel day as you only can deactivate an activated pass before 23.59 CET the day before the start date.

Not activating the pass in advance is especially important if you have a flexi pass as you can lose travel days if you have activated the pass and then get a late change of plans.

If you want to test that the activation of the pass works you can always activate the pass at home and the deactivate the pass again.

Ok thank you !

 We have a flexi pass (7 days within a month)

 We are especially afraid that it will take us a day of travel if we activate it in advance.

 We are sure on our first day of travel, that's why we want to activate it

In the activation process, you select the first travel day. After that, you can cancel that travel day and the pass is deactivated again.

Even if you would activate the pass, and thus select the start date of the validity, you are adviced not connect a journey from My trip to the pass until just before boarding the train. 

A journey connected to your pass can only be removed until 23.59 CET the day before the day of your journey.

If you connect the journey to your pass in advance and your travel plans for some reason, like a cancelled train or a strike, change on your travel day, then you loose a travel day