Every September for centuries, Icelandic farmers have rounded up their sheep from picturesque summer grazing grounds. On foot and atop trusty Icelandic horses, they join with family and friends to partake in a centuries-old event called réttir.

The annual sheep round-up is one of the country’s oldest traditions. Iceland’s 800,000 sheep spend most of the year roaming the country’s sublime hills and valleys, and each one needs to be rounded up. During réttir, waves of the wooly creatures cascade down the mountainsides, flanked by equestrians and pedestrians. They’re then shuffled into holding pens, where farmers read their ear tags to determine which sheep belong to which farmer. Once farmers have collected all their stock, they decide which animals to shear for wool and which ones to butcher for meat.

While réttir has a festive atmosphere that attracts increasing numbers of tourists, it remains hard work. Wrangling the sheep requires braving wet terrain and bouts of cold, cloudy weather. Participants spend hours in the saddle—for one day or several—and traverse switchbacks under the moonlight. It’s common for participants to share a flask to stave off the chill. 

That work pays off, though, when the event is capped with a celebratory réttaball. Communities gather for a night of traditional folk singing, dancing, drinking, and (of course) feasting on mutton dishes such as svið (sheep’s head) and lamb chops. And after the party ends, and as winter approaches, Icelanders can pull on warm, wool sweaters and look forward to next year’s réttir.