Come between November and April. Temperatures during this period range from around 25–30°C in the day to 10–15°C at night in the desert — comfortable for travel and cool enough to sleep well. January and February are the peak months: dry, clear, and stable.
March and April are still good but the heat begins to build. By May temperatures in the interior regularly hit 40°C and above. The hot season runs from May through October. From July to September a partial rainy season affects the south, and flash floods can make paved roads in Nouakchott impassable. The harmattan — a dusty wind from the northeast — can cause sandstorms in March and April.
Avoid the summer months unless you have a specific reason to be there and are prepared for extreme heat with limited support infrastructure.
For birdwatching at the Banc d'Arguin, November through January is best, when the migratory birds are at their peak numbers.