I had heard that the desert excursions (around Merzouga) only run during the cooler months of the year (November to February). But I've looked at a few sites and I haven't found anything that confirms this. Can someone please verify this with a reliable source?
Update: I picked up a copy of Rough Guide to Morocco and it states
Note, too, that the longer multi-day trips stop operating after February, after which time it just gets too hot.
Yet this contradicts what Karlson has posted, along with the fact that the websites for the excursions don't make any mention of this.