Junior level entry is usually combined with graduate entry for convenience of administration. You're probably familiar with the timetable from applying yourself, but for anyone who isn't:
September-November is when they mostly put out their recruitment materials. Applications open a couple of weeks thereafter. There may be a couple of ad-hoc positions, or smaller firms may advertise at other times of year e.g. January-March/June-July to catch postgraduates/undergraduates finishing their final exams. The largest firms may also be filling in last-minute vacancies if more people than expected turn down offers.
Otherwise, for round-the-year recruitment, the periods post-bonus announcements (i.e. January and April) are big times for moves.