This request involves the introduction of two periods, these so-called transfer windows, during which players can be bought and sold. You would need the exact weeks to implement this feature, so (in the proposed schedule I delivered earlier) the windows would fall under the following weeks (if we agree that the new season starts in July, as is the case w/ the majority of countries in Europe):
July-August period: weeks 1-9
January period: weeks 27-31
To anybody pointing out the odd number of weeks there, I'd like to remind you these are approximations since there are more than 4 weeks in a month (except Feb), and e.g. July+August=62 days, so almost 9 full weeks.
There remain a number of countries (a dozen or so) whose leagues run from spring to autumn, and this doesn't fit their schedules, but I don't know how to resolve that issue.