Winter when defined in this manner, can start on different dates in different regions. In terms of complete months, in most north temperate zone locations, winter months are December, January and Febuary, although differences exist from country to country. This is a Challenge cache and is at the posted coords. You will need to use the checker attached to see if you qualify. You must have logged a minimum of 700 caches in the winter months of which 5 of them need to be Multis and Letterboxes. All cache types count and no date restrictions. Byop.
Ground Fox has used Project-GC Challenge Checker to see if he/she qualified for this challenge and he/she did. Multi-caches found in winter (december, january and february) : 110 Letterboxes found in winter (december, january and february) : 53 Cachers found in winter (december, january and february) : 7072