Parisians take their vacations very seriously. They also have a LOT of vacation time each year - a minimum of six weeks - and some have even more than that. And they use it! Traditionally, the country almost shuts down during the month of August. The government has tried to encourage people to spread out vacations so that everyone doesn't leave at the same time, but the months of July and August are marked by huge departures. Everyone wants to leave the city.
Shops close. And suddenly storefronts sprout signs announcing dates of closure. Some "critical" businesses - like bakeries - have signs giving alternate addresses where you can buy your baguette. And yes, French people really do buy fresh baguettes EVERY day. Here is the sign on my "favorite" bakery.
Some shops take advantage of this closure for facelifts.
It can be somewhat exasperating to head for your favorite butcher shop just to find it closed for a month. But the good news is that I've discovered a new favorite butcher shop, new favorite fish market, and a passable substitute bakery as each shop rotates closures, often posting alternative suppliers. Everyone will be glad when all of the shops are open again.