The winter theme is picked up fairly early in the poem 'What lips my lips have kissed' by Edna Millay, with the discussion of the weather which we feel is just outside the poet's window: 'the rain is full of ghosts tonight.' We may pick up that the voices are ghosts from loves the poet once knewa long time ago, and therefore the rain is associated with present sorrow and sense of loss - even bereavement. The tapping of the window puts us in mind of wind, which again suggests that this is winter and suggests the presence of the twigs of a tree growing quite close to the house. This presence is then picked up in the next line 'thus in winter stands the lonely tree' where the poet's friends/loves from the past are likened to the leaves that have blown off the tree one by one. Compared to the evocation of winter, summer is given much briefer treatment,but poetic all the same 'summer sang in me' - this shows that the writer sees herself as barren and unproductive now that she expresses the song of summer no more. Compare with the poetry of Emily Bronte?
The barren condition of winter is heightened when set against the plentiful predicament of summer. The contrasts between both are fairly clear. The summer is a time of rejoicing under the plentiful presence of the sun, while plant and animal life are in full growth. In opposition, the winter is a time of emptiness, when all growth and animals have left or died out, and a cold presence helps to isolate all else from contact and connection. The poet taps into these imagery in comparing the predicament of the speaker as one who had experienced the joyful company of loves and others to one where there is a sense of loneliness and isolation. This is seen throughout the poem. I would pay attention to the lines 9-14 that help to bring this dichotomy to its fullest exploration. The growth and birds which populated the tree in the summer are no longer present, which is why the winter sees it as "lonely" and "silent."
This poem is about loss, and specifically about the loss that comes inevitably as we humans age. We cannot go back and feel the same things about the same places and the same people. Even if we see them again, we are not the same people we were when we originally had those experiences. So this is a poem about that kind of loss.
In poetry, of course, summer tends to represent youth and the best part of one's life while winter stands in for age and decline. This can be seen in this poem as well. Now that it is winter, the tree (or the speaker) can no longer know its past loves because the summer that used to sing in it no longer does.
So, now that it is winter, we are not the same people that we were in summer and we can never again be the same or feel the same as we did. The metaphor of summer turning to winter represents that decline and loss as we age.