Be careful: the deadlines to terminate a supplementary insurance are not the same as those of a mandatory health care insurance.
If you want to terminate your contract with your basic insurance, you will have to send them a letter before November 30. And it is not the date of dispatch which is authentic, but that of receipt.
However, if you wish to terminate your contract with your supplementary insurance whether it be capital insurance, hospitalization insurance or outpatient supplementary insurance the minimum notice period is of three months before the end of your contract. Some health insurance companies require a six-month notice period.
We will never say this enough, insurance companies can be of a great support in complicated moments, but terminating your contract is one of the most complicated things you will ever have to do.