Cap, winter, Service Dress.

They were floppy peaked caps (as opposed to the rigid ones worn at the start of the war), with fold-down ear-flaps that fastened under the chin to keep you warm in the winter. Introduced Winter 1914/15 (I think). Not liked by the higher-ups because they looked unsoldierly, and troops were only allowed to wear them in the front lines. When the tin hat came in, that became the regulation headgear at all times when in front line trenches, and because balaclavas could be worn under helmets to keep warm and Gor Blimeys couldn't the cap fell into disuse. Officially removed from service in 1917.
So late 1914 (or early 1915) to 1916 is when you would see fighting troops wearing them. Western Front, Salonika, Dardanelles, deffo. Maybe other places too.