[Someone]

Sheep shearing is highly seasonal; you shear your sheep in autumn when they have their winter wool.

Also, sheep herders might have a relatively easy task in summer. Herding might mean letting thousands of sheep loose in the spring, and getting them back in at the end of summer, not getting your sheep in every night or spending the night with your herd to protect it from predators.

Reading http://grapevine.is/news/2015/09/05/sheep-population-of-icel..., that's what happens in Iceland, too:

"The annual gathering of the sheep for slaughter – called the smölun (“herding”) – has commenced this weekend, and will be continuing for another month"