When there are at least 18 players from a country there will be a national team for this country that can participate in interlands and international tournaments such as the world championship.
Some days in the season are reserved for matches of the national team. No club matches will be scheduled on these days. If you play a friendly on one of these days you can't use players who were selected for their national team.
It's possible that a national team will not play a match on a day that was reserved for international matches, but in this case the internationals still can't be selected for friendly matches for their club.