Xjyuk

I plan to visit Paris from July 6 to July 9. I want to know what are the peak hours for public transportation in that time frame. When to avoid taking bus or trains, and when to take profit of low traffic.

Each city has it's own peak hours, and I want to plan my stay with those factors in head.

The short answer is that it doesn't matter. It won't make a noticeable difference to your travel time. Public transport is not like driving where rush hour traffic moves at a snail's pace. Buses can get stuck in traffic (even when there are lanes, French drivers often block them) but it's hardly ever slower than walking these days.

There are a few places where it's better to avoid traveling with bulky luggage at peak hours, but it isn't just a matter of time, it's also a matter of where. And parts of some lines are crowded outside peak hours because there are still many people traveling but fewer buses or trains.

Paris does not have any kind of reduced rate for off-peak travel.

Very very roughly, the morning peak in Paris is between 8am and 10am, and earlier in the suburbs: a lot of people start work at 9am or 10am. The evening peak starts around 5pm and lasts about as long. But once again, the effect depends on where you are. For example, if you're moving from a business district to a residential neighborhood during morning peak, you'll have frequent, half-empty trains, while the other direction is jam-packed.

You can get an idea of when peak hours are on a given line by checking its frequency:
buses and trams, suburban lines (RER, transilien). That usually won't tell you which part of the line is busy however.

Weekdays are Monday to Friday. Note that many attractions are open on Sundays and closed on Mondays. (Nationally, museum closing day is Tuesday, but in Paris it's usually Monday instead.) Most shops are closed on Sundays (but more and more shops are open 7 days a week, especially in touristic areas).

Many French people take a long vacation during the July-August period. However, that doesn't make a significant difference until around July 14.

One thing that's a lot more useful to know than traffic pattern is real-time traffic information. You can get it from the RATP or transilien website or from the respective apps (on Android, I use the RATP app inside Paris and the SNCF app for suburban trains).

In general, the peak hour in the morning is about 8:30 to 9:30, though this can vary a bit depending on which line segment and which direction.

I’d say the peak hour in the evening is around 17:30 to 19:30. Here there are probably even more local variations.

Note however that July and August have less traffic than other periods, and they cover the school holidays, and many people take holidays during that period. The lowest traffic is probably observed first half of August, followed by second halves of August and July.

But as those are school holidays, there are actually less trains/trams/buses than during other periods, so you could actually end up having more people per train/tram/bus.

The question, which has evolved somewhat given the comment, and could be now considered to be one of those requests-for-opinions questions, which the moderators don't like. I would have liked to add this as a comment but I don't have the points yet. If you like the answer you can put me over that threshold, hint hint hint.

If your desire is to maximize the experience of Paris on the street, her architecture, parks, street life, and the chance to examine at close hand and your leisure some spot of interest then I commend to you, as part of your studies of the French language, the word Flâneur.

The core of Paris is sufficiently dense as to make a set of walking tours, either just you swanning about, following routes from books which have routes marked out, or formal walking tours offered or some combination, worthwhile and alleviate the requirement of using public transport.

I know that the buses in off-peak hours will be sufficiently swift as to preclude any reasonable chance of much enjoyment of viewing what you can see.

Most museums in Paris would be madhouses in July, with the possible exceptions of the Musée de l'air et de l'espace and the Musée de la musique and I've found attendance on autumn Saturdays in the former to approach the level of annoyance.