For some trains the reservations will not be open yet. Which trains exactly are you trying to book?

Some trains, if not most won’t let you reserve seats or even buy/reserve the ticket more than 60 days ahead.

 Oh okay! For some of them it seems that they’ll only open 60 days before. 

I have the example of this one from Milan to Vienna. 

Do you think we should also book the seats in trains that it is not required since we’re traveling in July and August?

 Thanks for your help!

 Oh okay! For some of them it seems that they’ll only open 60 days before. 

I have the example of this one from Milan to Vienna. 

Do you think we should also book the seats in trains that it is not required since we’re traveling in July and August?

 Thanks for your help!

I think you’d probably be okay not reserving it, but it is pretty cheap, cheaper than buying the whole ticket, but I think if you did reserve it you’d more or less pay for piece of mind.

Carefull the Nighttrain Milano & Rome to Munich & Vienna wont operate this July due trackworks on the route between Udine and Villach :/

The Nighttrain from Venice will operate on a different route (via Slovenia)

I'm not sure why this train is still showing up in the Interrail reservations module. ÖBB have published their timetable for July (with engineering works) weeks ago and this one won't run.

There are still the night trains from Venice, Rome and Zurich, which give some options:

(Maybe leave a bit earlier from Milan to have more time to change in Venice.)

Or:

Or:

These trains are all best booked via ÖBB, so you don't pay any booking fees. Booking steps for night trains and for Italian day trains (also from/to Switzerland).

Thank you very much!! All of your tips were very helpful.