John Lightfoot on Luke 2:8

[And there were shepherds keeping watch over their flock, &c.] These are the sheep of the wilderness; viz. those which go out to pasture about the time of the Passover, and are fed in the fields, and return home upon the first rain.

"Which is the first rain? It begins on the third of the month Marchesvan. The middle rain is on the seventh: the last on the seventeenth. So R. Meier: but R. Judah saith, On the seventh, seventeenth, and one-and-twentieth."

The spring coming on, they drove their beasts into wildernesses or champaign grounds, where they fed them the whole summer, keeping watch over them night and day, that they might not be impaired either by thieves or ravenous beasts. They had for this purpose their tower to watch in, or else certain small cottages erected for this very end, as we have observed elsewhere. Now in the month Marchesvan, which is part of our October and part of November, the winter coming on, they betook themselves home again with the flocks and the herds.