What it is
Erg Chebbi is a sand sea about 22 km long, with dunes up to roughly 150 metres, on the eastern edge of Morocco near the Algerian border. Merzouga is the village at its foot. Camps sit out in the sand, reached by 4×4 or by camel in about an hour. The drive from Marrakech is 550-570 km over the Tizi n'Tichka pass and the Dades and Todra gorges, nine to ten hours without stops, which is why it is normally sold as a three-day, two-night loop.
Why go
These are the tall, clean, wind-shaped dunes people picture when they say Sahara. Erg Chigaga, further west from Zagora, is bigger and much less visited but needs a further 4×4 leg. The small dunes near Zagora that some short tours sell are not the same thing.
When to go
October to April. Nights from December to February can drop close to freezing, so take a fleece even though you are in the Sahara. June to August is above 45°C by day and should simply be avoided; heat exhaustion on a camel at four in the afternoon is a real outcome, not a warning label. Sandstorms are most common in spring.
Most people stay 1-2 nights.