The short answer
Friday is the day of the main weekly prayer, and lunch after the midday prayer is traditionally couscous in Moroccan homes. Families gather around one big dish of steamed semolina topped with vegetables and meat. For a visitor, Friday is the day to look for couscous on a menu, and to expect some souk shops to close around midday.
In more detail
Couscous is fine semolina, steamed over a pot of simmering broth with meat and root vegetables. Seven vegetables is the traditional number. It is served in one wide dish in the middle of the table, and everyone eats from the part in front of them. It takes time to make properly, which suits a day when the family is together.
Friday shapes the whole city. Many souk shops open late or close for a few hours around the midday prayer, then reopen in the afternoon. Not everything shuts. Many of the Mechoui Alley stalls rest on Friday, so plan your pit-roasted lamb for another day. A good Friday plan is sights in the morning, couscous at lunch and shopping in the afternoon.
In restaurants, many serve couscous only at the end of the week, on Friday and Saturday, so ask. Dar Couscous in the medina specialises in couscous and tagine. The Amal Center in Gueliz, a nonprofit that trains women in professional cooking, is known for its couscous on Fridays.
If you want to learn it, many cooking classes include a couscous dish, and the better ones show you how to roll the grains by hand.