A travel day is a 24-hour period during which you can travel on trains with your rail pass. It lasts from 12 a.m. (midnight) to 11:59 p.m. on the same calendar day—though, for overnight trains departing after 7 p.m., you need only enter the following day's date on your pass.
Within one travel day, you can take as many trains as you like and still be using only one travel day on the pass.
When you activate your pass, that's when the clock starts on its period of validity. If, for instance, you have a pass valid for five travel days in one month, your month begins when you activate your pass at the first train station.
Please note that you cannot add travel days to a rail pass. If you run out of travel days before the end of your trip, you'll need to either purchase a new pass in Europe (there are select stations at which you can buy certain passes, though they cost more than those purchased beforehand) or buy point-to-point tickets for the remainder of your travels.