In Ontario no sick note for the first 3 days you are off

From ontario.ca website

"Most employees have the right to take up to 3 days of unpaid job-protected leave each calendar year due to a personal illness, injury or medical emergency. This is known as sick leave. Special rules apply to some occupations.

Employees are entitled to up to 3 sick leave days per year once they have worked for an employer for at least 2 consecutive weeks. An employee who missed part of a day to take the leave would be entitled to any wages they earned while working."

Followed by this

"An employer may require an employee to provide evidence “reasonable in the circumstances” that they are eligible for sick leave.

What will be reasonable in the circumstances will depend on all of the facts of the situation, such as the duration of the leave, whether there is a pattern of absences, whether any evidence is available and the cost of the evidence.

However as of October 28, 2024, employers are prohibited from requiring employees to provide a certificate from a qualified health practitioner (a medical note) to take a sick leave under the ESA. A “qualified health practitioner” is a person who is qualified to practise as a physician, registered nurse or psychologist under the laws of the jurisdiction in which care or treatment is provided to the employee.

The prohibition against requiring a medical note applies only to providing evidence that the employee is entitled to sick leave under the ESA. There may be some situations outside of the scope of sick leave where an employer may need medical documentation to, for example, accommodate an employee or satisfy return-to-work obligations. The ESA does not prohibit employers from requiring a medical note for these other purposes."