Although too early to plan specific trains there would be no problem getting from Venice to Salzburg on a Saturday. The most direct route is via Villach or you can go via Verona to Innsbruck and then onto Salzburg.

Simply use the planner-or better a non-app like bahn.com or oebb.at for sameday next week-times will hardly differ-except when there are again works along somewhere.

Both ays can do and are in Austria even quite scenic. There is also an overnite train, but that will put you, if on time, down in Sb in the very early hrs.

There is a special 10€ (price now) surcharge for all the INTern trains from IT to AT/DE-can also pay on board, but they say then it will be more.

Thank you for the information! Would I need to make a reservation for this sort of trip or could my fiancé and I just take a regional train at the time with our global passes? 

Thank you for this information. Just to clarify, we will have to pay 10€ extra for taking a train from Venice to Salzburg even though my fiancé and I have global passes?

Yes unfortunately, the whole of the Italian section these EC trains cover is surcharged in this way.

You can also do either of the two routes Yorkie mentioned without paying €10 by using regional trains within Italy, but be advised that Villach is only served by surcharge-free cross-border trains twice per day, the first of which will be way too early for you. To catch the second of them, you need to Venice at 15:01 then change at Udine, arriving in Villach at 19:07. You would then have 9 minutes to change onto the IC train to Salzburg; unfortunately if you miss it, there’s a 2 hour wait before the next one arrives.

The alternative, which may allow for a bit more flexibility, is to go the other way Yorkie mentioned, via Innsbruck. This allows for you to take the lesser-known low-alpine scenic route through the Valsugana (although you can go via Verona instead if you like). To do this with no reservations (but a lot of changes!), you go from Venice to Bassano del Grappa, then to Trento, then to Bozen/Bolzano, then to the first station over the Austrian border, at the Brenner Pass. From there you can take whichever trains are convenient, as none will require reservation.

Okay thank you for this information! If we go the route where we pay the 10€, do we make a reservation for that and if so, how soon before should we try to make that reservation?

Ha, funny that you should happen to ask this today! As @Angelo could tell you, the best place to book the Venice-Innsbruck train is cd.cz (the site of the Czech national railway operator), because it is the only site that bundles in a free seat reservation with the €10 (or €15 in 1st class) surcharge. The price you see will be rendered in Koruna, so 251Kc. @seewulf’s instructions for using the website can be found here, on the first and second pages:

but if you get stuck, don’t hesitate to ask for more help. It almost certainly won’t be possible to book this train yet for May. All things being equal, booking a month or two ahead should be more than early enough, but you may be able to book it even earlier than that if you just want to tick it off the to-do list :-)

Innsbruck to Salzburg is an optional reservation, but you can also do this on cd.cz (75kc/€3.07 in both classes), among other places.

ohh didn´t notice that :D 

Only ÖBB askes for the reservation extra to pay. If you book via Interrail.eu/Eurail.com you get also the reservation for free on top because they book via SNCB/NMBS (B-Europe).  Also for free you get the reservation for this trains via ticket offices of SNCB/NMBS, DB, CD and Trenitalia. 

Thank you for your help with this!