It depends where you'll be applying to. Some carriers don't have any significant seasonal variation in numbers of flights. Charter and leisure ones tend to be busier in the summer, while for cargo the busiest time of the year is the run-up to Christmas. Recruitment policies also vary from company to company. Some carriers recruit with a longer lead time than others. For a charter job with a start date of 1 April or 1 May, one company will do the interviews before Christmas while another one will wait until February or March. So, I don't think that there's any "best" month to finish. Things line up differently for everyone.