What is a good temperature when mixing mortar for installation of a chimney?

live in madison, NH. Need a new chimney installed. Mason said he could do the job this week. Is it a good idea for now or later in spring...??? Is it convenience for him regarding his work load and need to get an job done>? So what about me??? Will chimney last with cool/cold temps in preparing mortar>? Or do I do this again several years to come.