The date was chosen by the Roman Catholic Church for various reasons, the winter solstice and the common sense of replacing a pagan holiday with a Christian Holy Day being one. If you look to animal husbandry for an answer then lambing in the free range environment of the time would have been in early Spring. You usually don’t need more than one Shepard to tend a flock unless an event (like lambing)is occurring, therefore the need for Shepard’s. Where I am from they popped out in the Spring and as soon as they were hopping about you “docked them” (put a band on their little tails, which would restrict blood flow and the little appendage would fall off -its done for sanitary reasons).