Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Sacramento to Nantes as you normally would.
Due to multiple factors, expect to potentially pay more when booking in March. For low rates, July can be a great choice for travel to Nantes. Flights to Nantes from Sacramento in July are about $1,179 on average during July, but can be found for as low as $671.