Just to be on the safe side, the landlord should really send it 7 to 8 months in advance. Indeed, if the certified letter with acknowledgement of receipt is not picked up by the tenant, the landlord will have plenty of time to have it delivered by a bailiff.

The letter shall contain the landlord's name, address and signature, even in the case of the property being managed by a real estate agency.
One of the three above-mentioned reasons shall be mentioned on it.

It shall be sent to the leaseholder(s) and:

To Mr. and Mrs. for a married couple.

Indeed, a notice given only to one of the spouses would not be valid. When the property owner is not informed that the tenant got married during the lease period, the notice given to the sole leaseholder is valid despite the married partner.

For partners in a civil union (a French PACS),

• if the lease is signed only by one of the civil union partners, then the landlord gives only notice to that partner;
• if the tenant signs a French PACS agreement during the lease period, one notice is sufficient as his PACS partner is automatically committed to paying rents, but not considered as a second leaseholder; 
• if both PACS partners have signed the lease contract, the landlord must sent two notice letters.

For co-tenants,

it is recommended to send a notice to each one of them.

Attention! Any notice letter sent by certified mail will be valid only if the acknowledgement of receipt is signed by the recipient. Courts consider that the reception date is that of the day of actual reception. If the tenant does not accept the letter or does not pick it up at the post office, whether intentional or not, the notice is not valid.

As a precaution, it is advisable to call upon the services of a bailiff. Even in the absence of the tenant, the bailiff can hand over a copy of the deed to any person being in the tenant's place of residence (the caretaker, a neighbor, domestic worker...) or, if no one accept it, the copy will be made available by the bailiff in his office (and no longer in the town hall).

The proceedings and bailiff office address are mentioned on the non-delivery notice left by the bailiff.
Then, the notice runs from the reception date of the bailiff notification.

Claire de Circourt