The validity period starts on your first travel day.

As you have a flexi pass you should be very carefully with activating your pass before your first day of travel. If you activate the pass before your first travel day and your travel plans for some reason change, then you can loose a travel day, as travel days only can be cancelled until 23.59 CET the day before the travel day.

You can test to activate the pass just to see that it works, but then you should deactivate the pass again.

Thanks! So just to be clear, do I make the train booking first in advance and then activate the pass on day of travel?

yes

Thank you so much

In the activation process, you choose the start date. But it's best not to activate a long time in advance because if you change plans and forget to deactivate, then you loose one or more travel days.

How can you make a train booking/reservation if your pass is not activated?

Via any means. There are lots of ways to book a reservation, none of which require an activated pass. Please check this thread:

If you still have questions, then please post (in a new thread) route, date and departure time of any train you would like to book.