You can add as many journeys to you pass as you want, just a few minutes, even seconds before you board a train. In fact, it’s very reccomendable to only add trains to your pass, just before boarding.

No need to register your trains weeks before travelling.

Although for some trains you will need mandatory seat reservations before boarding, and some of them need to be booked quite a bit in advance if you want to be sure of a seat (like Eurostar, and Night Trains).

How in advance do you suggest me to activate the pass if I want to reserve the seats on a train next Friday?

Make your reservations as soon as possible. You don't need an active pass for making reservations.

The recommendation from the experienced travellers in the community is to only make reservations through the Eurail website if no other options are available. Please read more about that below. 

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new topic, and you will get advice.

Please note that Interrail/Eurail charges an extra fee of 2 EUR per person and train in addition to the fee for the seat reservation.

 Activation of pass

During the activation process you choose the start day of the validity of the pass. Once the validity has started it can't be changed even if you haven't travelled. The advice is therefore to wait with activating the pass and starting the validity until the first day of your travel as you only can deactivate the pass no later than 23.59 CET on the day before the validity starts. If your travel plans change in the last moment you can't deactivate the pass and change the validity. 

It can be wise to make a test and activate the pass with a start date well in the future and then deactivate the pass immediately, just to see that everything works.

 Activation of travel day

The advice from the experienced travellers in the community is also never to activate a travel day, that is connect a journey to your pass and create the ticket (QR code), until just before boarding the train, otherwise you might loose a travel day if your travel plans change in a late stage You can't delete a travel day in the past. A travel day can only be deleted until 23.59 CET the day before the travel day.

Reservations are completely seperate from the pass. You cannot reserve in the app. The app is only used to register your trains, in order to generate a QR-code. It doesn’t “book” anything.  This QR-code the pass generates is your ticket. A ticket is NOT a reservation.

Luckily, most trains do not require a seat reservation. Usually high-speed trains (Frecce trains, TGV, AVE, Thalys, Eurostar, NIght Trains) or (international) long distance trains require seat reservations.

Seat reservations you can get via Interrail or via alternative ways. Check this page:

@Lisa Anzelini Which connections you are looking at to reserve? Please provide destinations, date, and time. Know that there’s usually multiple ways to avoid seat reservations. Only on Eurostar this is very difficult to avoid.

so my trip will be:

17th of march

Trento - Innsbruck Hbf

RE 1826  → h6.40 -9.38

Innsbruck Hbf - München Hbf Gl. 5-10

ECB 286 → h10.40-12.27

München Hbf - Berlin Hbf (Tief)

ICE 706 → h13.32-18.22

There is not specified that I need to make seat reservation on the app (Interrail planner), but it is high-speed trains so I guess there is the need to reserve something before activate the pass on Friday 17th right?

You don't need any reservations for these trains.