You obviously know that you must activate tomorrow or lose your pass.

Unfortunately your activation day is automatically a travel day and again it is use it or lose it on that day. To confirm tomorrow will be classed as a travel day even if you do not travel anywhere.

You will then be left with 6 travel days to be used at your choice and completed by midnight 30 September.

Above is right.
though recently-I think in the last week, I think I read some way around this, but as I myself (even now sitting in a DB- doubledeck IC, only 10 mins late, driver came in too late, using the wifi and on a pass-paper) never used mobile, I cannot repeat it. Maybe a search helps.

If I recall correctly that doesn't need to be the case if you do the activation procedure TODAY (before midnight CET, still 1 hour left) and cancel the travel day again without deactivating the pass:

1. Activate the pass and choose start date 31 August. That day will automatically be activated as a travel day.
2. Go to My Pass, select your pass and then select the activated travel day. Then tap on Cancel travel day.
3. The app will also ask whether you want to deactivate your pass, but then choose No.

You should then an activated pass and 7 unused travel days.

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Thank you for your replies! Unfortunately i just read them now..so the option with canceling first travel day again is now probably gone :(

I didn´t know that the activation day is automatically the first travel day. I thought i can still choose the first travel day within one month.

So probably i will lose one travel day tommorow when i activate my mobile pass since i cannot start traveling tommorow.

Maybe there´s still an option of exchanging my pass?

If your pass is exchangeable (check the invoice in the conformation mail), then you can do that for a 15€ fee:

https://www.interrail.eu/en/interrail-passes/ordering-info/refund-exchange-policy

It's not clear to me though what the timeframe for an exchange is (for refunds it's clear).

But what I'd do is ask customer service to give you back the lost travel day (after it's over). You should always be able to activate a pass without immediately activating a travel day. IMO it's a bug that it currently works this way.