You can join an English course every Monday. Courses are designed in a way that allows you to decide the number of weeks to book. While the minimum stay is one week, there is not a maximum limit. That is why the price structure is per week, and the more you book, the cheaper is the fee per week.
Long term bookings are bookings for a minimum of 12 weeks.
While the above applies to all the General English courses, some courses, depending on the school, will be available on specific dates only.