Friday is the Muslim holy day, and Moroccan families traditionally gather for a big lunch of couscous with seven vegetables after midday prayers — many restaurants build their whole week's menu around it, so it's the best day to seek out an authentic couscous plate.
It's also common for smaller, family-run shops and some offices to close for an hour or two around midday prayer time on Fridays, particularly outside the main tourist districts. This is rarely disruptive for travelers, but if you have a specific errand — exchanging money, a artisan workshop visit, a government office — it's worth checking hours or simply planning around early morning or afternoon instead.