Masonry in the northern states - what issues are there for doing in the cold? - seems like always try to get of it.

Masonry in the northern states - what issues are there for doing in the cold? - seems like always try to get of it.  I worked on a case that claimed "un-safe conditions" just to push it to spring, others insist on pushing to spring - does it cost that much more, is it just the power of union, ie guys off or claiming un-employment?  Or is is done in winter all the time, and just a coincidence.  The project was extended, simply from the owner not needing it right away - not because of a delay, but the contractor added a calendar for Masonry starting March 13th.  Does that seems reasonable.  Other than it being unfortable - the only other issue is heating water correct?  Not sure what else you have to heat or at what temp. it is too cold for masonry?