The short answer
Throne Day, on 30 July, marks King Mohammed VI's accession in 1999 and is one of Morocco's most celebrated national holidays. In Marrakech you see flags and lights strung across the avenues and a festive mood; the royal address is broadcast and the formal events take place elsewhere. Offices close, but riads, restaurants and sights stay open.
In more detail
Our holidays guide calls Throne Day the biggest fixed holiday of the year. It is a fixed date, so it falls on 30 July every year, unlike the Islamic holidays that move with the moon. Expect decorations in the days around it and a lively evening in the city.
For a visitor, the practical effect is small. Banks and government offices close for the day, and some shops may keep shorter hours. Riads, restaurants and the main sights carry on, and many join in the mood.
It falls at the hottest time of year, with July highs of about 37 °C, so any evening events are the ones to catch. If you are in Marrakech then, plan the day the summer way: sights early, rest in the afternoon, and out again after sunset, which is at about 19:40. It is a proud day for many Moroccans, and asking your guide about it is a good way into a conversation.