I only read Dickenson a long time ago and only recall some of her poems but mostly her personality. I forgot that and read this a few times. What is meant to me was the pressure of time and within it, tragedy. Coldness in feeling but then heat and warmth as memories come to me before the cold of Christmas, it ends in me asking will I be able to recover a memory before that of some guy who caused me to weep in the frock I wore then. I.e. I want to wear the frock again as if it were all new. I tried to penetrate beyond that and came up with: I am pregnant, I have nobody and am depressed about it. I wondered if the cheek was a butt cheek of that guy who whizzed past me on the ice. I felt that if pregnant I might be wondering if I will have a boy or a girl. The bridge disturbed me, was I going there to commit suicide? Or was it to get closer to the guy because he isn't there? These are just my impressions.

How can you be "tarnished by the snow?"
Is it its coldness? Compared to the warmth of "a summer's day."

I don't understand the second half: sings, speaks, weeps. The border of the hems and the bridge away.
"Before the ice is in the pools" - before her weeping becomes fixed, becomes solid. While it is still warm and changeable, will still change come about? Before a barrier is created.
"Before the fields have finished" - before the blossom has gone, and perhaps the new beginning of the Christmas tree.

"What we touch the hems of   
  On a summer’s day;"
- A frock

"That which sings so, speaks so,   
  When there’s no one here,—    "
- Her heart?

I like her coloring. The cheek tarnished by the snow you feel the redness of it, the whiteness of the snow. The greens of the fields and the Christmas tree. Her white or blue (color of water) frock.

Why is the first reaction when she describes her weeping to think of a guy? She says:
"Wonder upon wonder
Will arrive to me!"

Do we really think of Emily Dickinson as only interested in guys and weeping only because of them?
What sadness does she want to pass? What revelation?

To me it is a poem of great amazement and wonder.

As Chris nicely describes, a moment just before something is about to happen. Before crystallization. In place of the expected miracle of the Christmas tree, she experiences a private miracle. The borders, edges are felt.

There is a private, personal access to a feeling of wonder which one brushes up against in nature in the green of summer. Society celebrates the formal ritual of Christmas in a setting of ice and snow, while she prefers a no less real miracle felt in the living earth.

I like the double meaning of "before" -- prior to and in the presence of.

Thanks, Solveig. I look forward to understanding more.