On an Interrail or Eurail Pass, a "travel day" is defined as a 24-hour period from 00:00 (midnight) to 23:59 on the same calendar day. During this window, you can board as many trains as you like within the participating European rail networks. If you have a "Flexi Pass" (e.g., 7 days within 1 month), you must manually or digitally "activate" a travel day before boarding your first train of that day. A critical rule for 2026 is the "Direct Night Train Rule": if you board a direct night train that departs before midnight and arrives after midnight, you only need to use one travel day—the day of departure. However, the date of departure must fall within the overall validity period of your pass. If you change trains after midnight on a night journey, you must then use a second travel day. This allows for cost-effective "long-haul" travel across borders while maximizing the value of your selected travel days.