While snowfall is not as rare in the African continent as you may think (there are some ski resorts in South Africa, for instance), the white stuff is certainly a extremely unlikely weather event in the Sahara Desert.
But as it so happened, snow fell on the Sahara on December 19 last. It stayed for a few hours, then melted away. Nevertheless, the event was significant enough to make headlines worldwide.
Covering an area comparable to the United States, the Sahara is the largest hot desert in the planet. Very little rainfall occurs year-on-year, let alone snowfall.
According to records, the last -very brief- snowfall occurred in 1979.