Don't know if I need 5 or 6 Day Euro Select Pass?

The night train, if it leaves Zurich after 7 PM and you stay on it until 4 AM, counts only as the following day. The train from Interlaken to Zurich counts for the previous day. Two days. You might look at the cost of a point-point ticket from Interlaken to Zurich and see if it's less than a sixth day of the pass.

Also check Austrian Rail. They have pretty good advance purchase fares between Austria and other countries, like Zurich to Vienna via Innsbruck, and Vienna to Venice via Villach (bus from Villach to Venice). Check Trenitalia.com for Italian fares, Venice to Florence to Pisa, Pisa to Rome. You might not even need a rail pass.

Typo alert. It's boarding by 7 pm and the trip lasting until after 4 AM.

Swan, you're right, typo.

thanks guys!

I checked it out and getting a 6 day Euro [INVALID] pass would be cheaper than sticking with the 5 day and buying an individual point to point from interlaken to Zurich.

I like to take advantage of those inexpensive extra days when I buy a flexi pass. Then I can justify using the pass for a couple of less expensive train trips, and they are usually non-reservation type trips. I can just hop on a train and go, on a day-trip or whatever.