What is a travel day?
You can buy a pass with either 3, 4, 5, 6, 7 or 10 travel days. A travel day lasts for 24 hours, from 12 midnight on one day to 12 midnight of the following day. On each travel day you can take as many trains as you want. The travel days can be used within a 1 month period.
If you take a night train that leaves on one travel day and arrives on the next, you only have to write down the date of arrival in your travel calendar.