Early in the morning. Assuming that you want an outdoors shoot with some iconic Parisian architecture in the scene, you won’t be competing with hordes of tourists and you’ll get the golden hour light too. Of course you might have to work around delivery vehicles but it won’t be nearly as hectic as it will be later in the day.
As for the time of year, Paris doesn’t really have an off season. During August, all the locals go to the countryside and a lot of places close but the tourists keep coming year-round and the tourist areas will always be open.