If you order an Interrail Global Pass - 5 days within 15 days, this means you can choose any 5 days to travel on within a period of 15 days.
The travel days do not have to be consecutive. You can decide when to use them during your trip. A pass like this is ideal if you don't plan on travelling every single day. Below are your options:
5 travel days within 15 days or 7, 10 or 15 travel days within 1 month.
3, 4, 6 or 8 travel days within 1 month.