The McGill TA positions opened up on March 15. The applications are due April 15. You can see the postings here: http://www.mcgill.ca/music/current-students/graduate/job-postings-0 These spots are competitive and I know some current students who have never had any TA positions despite applying every year - a lot of it has to do with your financial status - if you get a lot of fellowships from the university you are less likely to get a TA position. At the Master's level you can receive TA spots for a maximum of 5 terms.