Most group health insurance plans are "unilateral contracts". This means that employers can cancel a group health insurance plan at any point during the year. While most carriers “request” 30 days notice, this is not always required.
For the smoothest transition, your group plan cancellation date should be far enough in the future (usually 30 days) to ensure that employees have a reasonable amount of time to find an individual health insurance plan. Most insurance companies will allow individuals to back-date their policy to the first day of the month in which they apply.
Call the insurance company for more information on their cancellation policy.