Is it permissible for an employer to cancel employees' official holidays because they conflict with Islamic Sharia, or to agree with them to cancel them, and does this ruling apply to Muslims and non-Muslims alike?

It is permissible for an employer to agree with an employee on determining annual or weekly leaves, even if they differ from what is stated in labor law, if there is mutual consent between them. The decisive factor is what they agreed upon in the private contract, based on the words of Allah Almighty: "O you who have believed, fulfill [all] contracts," and the words of the Prophet, peace and blessings be upon him: "Muslims are bound by their conditions." If they agree that the employee is not entitled to leave on certain days and the employee consents to this, then that is what is considered valid.