Alternate day off selection for public holiday in ON 2022

When two public holidays falls consecutively and one is on a weekend and the other on a regular workday, can an Ontario employer select an alternative date either before or after the holiday? For example, Dec 25, 2021 falls on Sunday and December 26th falls on Monday in 2022. Can we select either December 23rd OR December 27th as an alternative/substitute day off in Ontario?  Does the employee have to agree in writing, or can the employer unilaterally decide?
Thank you in advance for your information.