Jan 11, 2011 - 01:52 PM
A travel day is considered any travel between 12 midnight on one day to 12 midnight of the following day (i.e. a 24 hour period). On each travel day you can take as many trains as you want.
An additional benefit is the '7PM rule': If you board a direct overnight train that departs after 7pm and arrives after 4am, then only the following day (the day of arrival of the night train) is counted as a travel day. On your Eurail Pass, you would write the day of arrival on the pass prior to boarding the overnight train