CRA has clarified that:
For calendar year 2020, May 31 falls on a Sunday. As such, any filing or payment deadline that would normally fall on May 31 will be treated instead as a Monday, June 1 deadline. Furthermore, any extension that applies to a June deadline will also apply to an original May 31 deadline.
Also note that the extensions to the filing due date for most income tax returns in the deadline chart also apply to form T106, T1135, and any elections, forms and schedules that must be filed with the return.