What is the "separate peace" that funny has established?

One of Finny's most creative undertakings is Devon's Winter Carnival. As Gene pointed out, Devon did not have a Winter Carnival, which did not deter Finny at all in putting one together to break the monotony of the gray New Hampshire winter. After smuggling materials out of their dorm (including some jugs of hard cider), the boys established the site of Finny's most recent adventure in the small park beside the Naguamsett River.

Once the Winter Carnival was underway, the boys lost themselves in fun and games, literally, with some help from the cider. All thoughts of war were banished for the afternoon as they simply enjoyed the freedom of being young and alive. Gene remembered what he felt that afternoon, and his feelings were shared by the others:

It wasn't the cider which made me surpass myself, it was this liberation we had torn from the gray encroachments of 1943, the escape we had concocted, this afternoon of momentary, illusory, special and separate peace.

Finny's Winter Carnival creates for himself and the others an illusion of peace; for a little while, it takes them away from the reality of war, death, and ever-present fear. Very symbolically, it is Leper's disturbing telegram that brings the boys' joy to a sudden, sobering, and unexpected conclusion. They cannot escape reality for very long.