What is a travel day?
A travel day is a 24-hour period in which you can travel on trains with your Interrail Pass. It lasts from 12:00 AM (midnight) to 11:59 PM on the same calendar day. On each travel day, you have access to the train networks where your Interrail Pass is valid.
On average, it won't take you more than 1 travel day to go from one European city to the next. For instance, you can use 1 travel day to get from Berlin to Paris and another travel day to go from Paris to Madrid.
With a Continuous Pass, such as the 1 Month Continuous Pass, every day in the validity period is a travel day, so you could travel every day if you like! With a Flexi Pass, such as the 5 days in 1 Month Pass, you pick and choose which days you want to travel up to the number specified.
When travelling on a night train that leaves on one day and arrives the next, you only have to spend 1 travel day: the day of departure