The speaker is living away from home, in a place he calls "exile," and the only time that feels real to him is the weekend, when he can return. The rest of the week seems like something to get through. Because he doesn't want to look ahead at all the time he still has to spend where he is, he finds a different way to measure it: he counts the weeks backward.
That small change turns each passing week into a small gain. Every week that slips by means "one less" in exile, so time that once felt like a burden becomes a kind of progress. The mood is quiet and weary, with a thread of homesickness running through it. The short, broken lines near the end, especially the repeated "In exile," make the loneliness feel heavy and the comfort feel thin. The speaker isn't celebrating. He is consoling himself, and the poem lets us see how little it takes, and how much it costs, to get through a stretch of time you'd rather not be living.