When I was teaching in colleges and universities, April and May were the best time to be contacting employers and asking about their openings for the autumn semester. October (right after the holiday) to about mid-November was best if you were just looking for a spring semester. That said, things change and sometimes openings come up at the last moment. I remember one job I got, I contacted the college the second last day before the winter holiday just on the off-chance they needed someone. They did, because the teacher they thought they had hired had a problem at home and couldn't leave for China.