Notice runs asymmetrically: the employer's obligation grows with length of service while the employee's generally does not, and a collective agreement can override both. Anyone planning a resignation date, or reading a termination letter, needs the right scale before working out when the contract actually ends.
Scales with how long you've worked there: under 5 years gets 1 month, 5–10 years gets 2 months, 10–15 years gets 3 months, and 15 years or more gets 4 months. Your own notice period as an employee, in the other direction, is always 1 month regardless of tenure, unless your contract explicitly says otherwise. A CAO (collective labor agreement), if one applies to you, can override these statutory numbers, so that's the first place to check rather than assuming the default scale applies. Notice generally runs to the end of a calendar month unless your contract or a CAO says differently.