Eurail passes are available in a range of travel day combinations. A travel day lasts 24 hours (midnight to midnight). On each travel day, you can go by train in the countries where your pass is valid.
If you plan to travel by train most days of your trip, a continuous pass gives you the freedom to travel every day if you want. Example: 21 days continuous.
If you want to travel by train on a set number of days, pick a pass that lets you to choose when you travel over a given period. Example: 10 days within 2 months. Decide on the go when you use each travel day during the period your pass is valid.