Torah requires its adherents to sacrifice animal every day, morning and twilight.
Now this is what you shall offer on the altar: two one year old lambs each day, continuously. The one lamb you shall offer in the morning and the other lamb you shall offer at twilight; (Exodus 29:38-39)
This verses created a real need of two lambs (young sheep) in Jerusalem every day in any weather, including December weather. However, citizens were prohibited from raising sheep inside Jerusalem.
One may not raise small cattle [i.e., sheep, goats, etc.] in the Land of Israel, but one may do so in Syria or in the uninhabited parts of the Land of Israel (Mishna Baba Kama vii 7)1
The sheep farming, which are located close to the highway to Jerusalem, on Bethlehem pasture supplied and met the need.
- The sheep farming on Bethlehem pasture was always there, so were the shepherds, to meet the need of daily sacrificial sheep in any season, weather, month, and temperature.
- Here is a reason for the presence of shepherds and their sheep on Bethlehem fields in December.
1 http://www.jewishvirtuallibrary.org/jsource/judaica/ejud_0002_0005_0_04581.html. January 9, 2015. 11:49