In the winter, shepherds don't sleep outside with their flocks; they herd them into shelters, such as caves. So do women giving birth. indoors, not under the winter sky.
The Christ Child was born when the Shepherds move their flock of Sheep with lambs.
That leaves in a logical sense, either,
A) moving from from winter grounds to greener pastures (ie: Passover)
B) moving the flock from Fall to Winter
Falling upon 3 days after the Winter Solstice, is a way to convert Northern Hemisphere
peoples that related to the Winter Solstice, yet understood the significance to the Child of Light,
that might culminate each year in the Summer Solstice
Scripture call John the Baptist being born at Passover and Christ 6 months later. Do the math.
Oh...and the Bible is based on a 10 month calendar, not 12 months when the Catholics changed it.
Sept, Oct, Nov, Dec =
7, 8, 9, 10.