The shortest daytime connection is 6h 29min; but that’s not without extra charges; without any charges it’s 10h 13min - e.g. 13.37 from Zermatt, change in Visp, Brig, Domodossola, Milano and Verona, arrives 23.50. There are many other connections that take a little bit longer. 

Doing this overnight is a strange idea because it does mean a long stay (a few hours) in the middle of the night at a station (there is no direct night train). It’s possibe to do this in about 15h without any reservation or in about 12h with mandatory reservation (that comes with a fee).

To give you an idea: 

But I don’t think this answer will help anyway. You didn’t provide any travel details. Connections do depend on the time of week and the time of year (e.g. the direct route by train is closed in summer). I assume that you did look for a date too far away when getting no results at all.

Thanks for your response, Hector!

Yes, we were planning to travel on 22/07/2024. 
We would prefer travelling over-night.

Say for example,  Depart at around 9 PM and arrive in Venice early morning. 

I direct train route is closed, what would be the alternate options using Eurail?

This route is shown above (departs 22.13). It’s possible to arrive in Venezia by reservation-mandatory train at 9.42 (at least now; for July timetables you’ll still have to wait a few weeks).

There will be a bus replacement in summer (between Domodossola and Milano), but there’s no timetable yet. So it will possible to go the direct route, but on some part by bus.

Otherwise it’s possible to go via Lugano to Milano. But timetables for Italy in July aren’t published yet. But it’s not a comfortable journey at all - possible connection: 19.13 from Zermatt to Bellinzona - 3 hours in the midle of the night in Bellinzona - 10.20 in Venezia Santa Lucia.

There is no night trains with sleepers/couchette on this route if that's what you are looking for.