Hi, Linda! I think 1917 or 1918 is a really good guess. It is probably not Halloween though. The way we celebrate today with children in costume and trick-or-treating came into vogue in the 1930s. It may have happened before but usually in spooky costumes. There were lots of other occasions when people dressed in costumes - New Year's, Easter, Christmas, to name a few. With the snow, I wonder if New years would be the occasion. Men in uniform might have been plentiful since we entered WWI 2 April 1917. Yeah, I know, the historian in me just won't shut up!
This hat - "cover" in military parlance - would not have been worn in a combat zone. In WWI it would have been the "dough boy" steel pot (sidenote: This combast helmet earned the nmae dough boy after Salvation Army lassies started making their doung for fried doughnuts in these helmets IN THE TRENCHES!). The Campaign Hat was originally worn overseas in camp or garrison but quickly replaced with the peaked cloth hat ( the tent-like one) we know today because it was difficult to care for and space was limited, hence is became a hat strictly for in Garrison or on post waer. The Campaign hat has been worn continuously since the 1840s when it was unofficially adopted by troops sent to the West. It was officially adopted in 1870 and has been worn continuously in varying iterations since and by all ranks, including Generals, by the US Army, Air Force, and Marine Corps. Today it is worn only by Drill Sergeants and Drill Instructors in the Military. Several other official and unofficial units also were it. Smoky Bear and the US Forestry Service, Boy Scout leadership, the Royal Canadian Mounted Police, etc.
OK, I tell you all that because I never really left the Army - that oath is for life - and because the history of men's clothing changes slowly compared to women's fashions and this hat is exceptionally useful in dating men's photos.
