With the consent of your supervisor or manager, who may not refuse without a valid reason, you can switch your annual vacation with a coworker.

The excerpts below from the contracts for the three major McGill University Health Centre (MUHC) employee unions, explain the procedure for their respective members.

Note: This applies across Quebec for all APTS, FSSS/CSN and FIQ health and social services sector union members.

APTS – Once the employer has posted the annual vacation schedule, two employees with the same job title, in the same centre of activities may exchange their annual vacation period with the agreement of the supervisor.

MUHCEU FSSS/CSN – Two employees with the same job title, working in the same service and entitled to the same number of days of vacation, are free to exchange their annual leave with each other, with the consent of their immediate supervisor, who may not refuse without a valid reason.

SPSICR-CUSM FIQ – By mutual agreement, two employees of a same centre of activities can exchange their annual vacation dates, with the agreement of their immediate superior.

Source: Local Contracts 
APTS Local Collective Agreement
MUHCEU CSN-FSSS Local Collective Agreement
SPSICR-CUSM Local Agreement