Roshni - there is a very clear guideline within every Organization's resignation/ termination policies - please read through the clauses again. Typically, a resignation must be given during business days and within business hours with a written form of communication, e.g., an e-mail. Certain Organisations do not even consider resignation a valid one if it is sent from a personal e-mail ID.
Advice - please refer to your Organization's termination/ resignation guidelines as to what must be accepted as a 'valid' resignation.
Utmost advice - Please do not confront an employee with any form of hostility, it is not an ethical practice anymore and will never be appreciated. Please make the employee comfortable and communicate with ease that the resignation would be deemed effective from xyz date due to xyz policy.
Thank you.