The academic year is divided into two semesters of approximately 20 weeks each. Each semester is divided into two quarters in approximately 10 weeks periods.
The autumn semester generally starts at the beginning of September and runs to about 15 January (quarters 1 and 2).
Spring semester starts about 15 January and ends first week of June (quarters 3 and 4). Every quarter ends with an exam week.
Make sure that you select courses that are in the correct semester and quarter in order to have a balanced course load. Also make sure, if possible, that the courses are in different modules to avoid collisions in the timetable.
30 credits corresponds to one semester full-time studies.