For 2026 rail passes like Eurail or Interrail, a "travel day" is strictly defined as a 24-hour period from midnight (00:00) to midnight (23:59) local time. Within this window, you can board as many trains as you like on the participating networks. If you are using a "Flexi Pass" (e.g., 5 days in 1 month), each day you board a train will deduct one day from your total. A critical "money-saving" rule for 2026 is the "Night Train Rule": if you board a direct night train that departs before midnight and arrives the next day, you only have to use one travel day (the day of departure). However, if you change trains after midnight, the system will automatically count a second travel day. For mobile passes, the app uses your phone's clock to validate the day, so it is vital to ensure your time zone is correct when crossing borders (e.g., traveling between Poland and Lithuania) to avoid accidentally "starting" your next travel day an hour too early.