Paris in July vs August

1) What week in July do Parisians typically leave en masse on their vacations?

The very Last week in July and 1st week of August

2) What week in August do Parisians typically come back from their vacations?

The last Monday in August or first Monday in September I know because I listen to the national public radio programming from France and the broadcast season which runs (September to June 30) they call the La rentree (the return) always starts then.

Just go the first or second week of September for less tourists

1. Many Parisians leave around the fete Nationale
2. The Parisian return is called naturally the rentree which varies according to school start dates at the end of August, beg. Sept.

Rest assured, Paris does not empty of Parisians. Who do you think is waiting table, selling the tickets, making the beds, driving the buses? Although Paris can be very quiet in some areas in August, you probably wouldn't notice as you'd (again) probably be staying in central Paris where there are fewer and fewer Parisians regardless of the season. No matter the time of your visit, if you're looking to have the company of Parisians, you\d want to stay in one of the outer arrondissements

''Just go the first or second week of September for less tourists"

September finds Paris jam-packed with tourists.

A long time ago working-class Parisians went back to the countryside to visit families in August, while wealthy ones went to beach homes, but as said above, those days are long gone. First, today only 50% of Parisians can afford to leave Paris at all. Second, vacations are spread out from May through September, September being the gray-hair season. People come and go all the time during these months. Nothing is set in stone.Finally, as relatives who lived in the countryside have died off and modest houses have passed out of families, lower-income people can no longer afford to go away for a month paying for a rental, so vacations away are often less than a month: one, two, or three weeks. Therefore, there are plenty of Parisians in Paris.

On the other hand, If you stay in wealthy neighborhoods, such as rue Cler (yes, this is an old-money neighborhood), the 16h, the 8th, or parts of other arrondisements on the west side, there will be fewer Parisians floating around because a portion of these residents can afford to go away or to second homes. The Ile St. Louis is empty of permanent residents most of the year, as so many of those apartments belong to people who live elsewhere, but no one ever complains that it seems deserted, as the lines for ice cream go around the block.
Bottom line is that if you are going to tourist attractions, it's not going to make a lot of difference. If one restaurant is closed, ten others will be open, the museums and parks are open, and all will be well. BTW, what are those sources? They have simplified and generalized.

Edit: my French husband reminded me today that indeed, when there were factories in the Paris region (Renault, Citroen, etc.) they would all shut down in August so everyone could take off. Some offices were down to a skeleton crews or closed. Employers sponsored more camps, camp grounds, and group vacations. However, this happens much less in this post-industrial society.

I think the reason you see different dates is because people do come and go at different times.

Our experience last August/September (our 2nd visit in this time) was in the 8th arr. in an area that's not a big tourist magnet. The neighborhood restaurants/shops were staggered in their vacation time in August. We wondered why we had a sub-par meal at a previously favorite restaurant only to see they left for vacation the next day. Maybe they were cleaning out the fridge?! When we came back the first week in September there was a noticeable increase in business people and families navigating their way to school. We actually enjoy the more relaxed "vacation" feel as people seem more inclined to chat.

Whatever you want to do or see in Paris will not be impacted by people who live there taking their summer vacations. All attractions will be open and you won't find too many restaurants or shops that actually close anymore. Don't even give it a second thought. We usually go there at this time of year and will be going for a month this summer again. It's never been an issue.

There are over 2 million people living within the city limits and another 8+ million in the surrounding suburbs. Most of them don't spend much time in the center, where the tourists are. Many of those that do are commuters who you won't see either. In fact, with many on vacation and staying home (not every Parisian can afford to go away for a month), you may see more of the locals out and about, enjoying the parks, museums, and cafes, maybe some of the other 50 million Frenchmen will be vacationing in the city. Who knows, maybe even less rush hour traffic. As far as shops and restaurants closing, I doubt you'll notice much of that in the center either. On the other hand, September is a huge trade fair month and many business people are in town, which means a much greater demand for rooms.

Parisiens go when the idea strikes them - and when they can get a great deal.
I'd go in early July.

Make sure you get a hotel with air-conditioning. Rental apartments do not have them.

Hotel rates sometimes drop from mid to late july through August.... not because of locals going away.. but because business travel dips a bit and so some hotels put on deals.

There is no way you are having a "more authentic experience " going in any month from the other.. I've been in many months and its just not going to affect you ( unless you have a special restaurant you want to see.. and even then.. they dont all close for a month like they used to.. many are only closing for 2-3 weeks.. ).. I mean you will be going to tourist sights like the Eiffel Tower and museums right.. and those are always busy with far more tourists than locals.. One advantage of going in summer is you can avoid the school groups that are often visiting museums.. they can be surprisingly disruptive.. lol

And one of the reasons Paris Plages was started was NOT for tourists but for locals who couldnt afford to leave city in late july or august to have a beach experience.. ( Paris Plage is a fake beach they set up along the banks of the Seine )

I'll just add to make sure your hotel has AC during this time frame.

I second Pams advice on air conditioning and will add that hydration is very important.

Thanks so much for all the comments!

@ Bets - One source that confused us was the "Join Us in France" podcast... an old episode on "July and August in France".

Thanks Ella. I love Join Us in France and find Annie and Elyse very informed, to say the least (PhD in art history—Elyse really knows her field). But as you said, it’s an old podcast and both women have spent little time in Paris, but live in Toulouse. I see they have a podcast this week about August in Paris, and the written intro says that things have changed. We’ll all have to listen.