One free day per season? That will be a big change, people are used to at least one game a day. Except Nordeus is constantly having overload issues which may necessitate free days for events.
One free day per season will work well if there are special benefits like free training without the need for greens, so we can train without fatigue (usually sybolised with the green energy bar turned red). This means a 24hr period where the green bar remains constant. It bears resemblance to reality when real clubs train before the hectic season.

If Nordeus should create a free day that has no activity for players or users, I guarantee you that they will gradually loose their fans and fanatics.