“A travel day” is a day that starts at 00:00 and ends at 23:59.
If you leave Cologne at 08:00, get to Basel and then take a TGV to Paris at 14:00, and then take a local train to Brest at 18:00 then this is all still one single travel day. It is a day that you are traveling on.
The fact that it includes your “own country” day from Cologne to Basel is incidental.
If you were to stay in Paris overnight and get the first train out to Brest at 06:00 the next day then this would be a second travel day. The fact that you started the day before at 08:00 and took your last train within 24 hours doesn’t count for anything. The travel day ended at 23:59 the day you left Cologne.
Now, if you got the night train to Nice and left Paris at 21:00, arriving at 08:00 the next morning this would still count only as a single travel day - because you boarded your last train before the end of the travel day.
There are no sleeper trains to Brest though . . .