Traveling Days - does it have to be the first and last day of your trip?

Hi everyone,

Saturday, I'm going to start on a 15 day-trip (in 2 months) with an Interrail pass. This will be my fifth year I'm going to travel by train, but I have a question.

On 1 of your travel days you're allowed to use the pass to travel out of your country, and on another day you're allowed to travel back in. Does it always have to be the first and last day of your trip, though? Or are you allowed to use any day in your travel period?

Thanks in advance!