At http://www.biolreprod.org/cgi/content/abstract/25/1/85 is
an article on how the springtime is considered the idea season for lambs to be
born , with Fall being the "wrong" (undersireable) time of year for
them to be born.  The idea that "Christmas" is the normal time for
lambs to be born is just simply wrong.  Lambs CAN be born at any time of the
year, but at http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=7174545&dopt=Abstract
is another article that compared what happens when Lambs aren't born in the
spring.

http://ars.sdstate.edu/sheepext/sd99/99-1.htm also says
that spring is the more normal time for lambs to be born.

Spring is great.  Lambs for Passover are suppose to be a year old.  So if they
are born in the spring, they have a chance to become 1 year old, and used in
Passover just as they reach that age!