Therefore all seasons shall be sweet to thee, Whether the summer clothe the general earth With greenness, or the redbreast sit and sing Betwixt the tufts of snow on the bare branch Of mossy apple-tree, while the nigh thatch Smokes in the sun-thaw;
Since God instructs humans through Nature, says Coleridge, every season has something to say. You can't say that winter is totally horrible, by these rules—even if you're not an ice-skater or a snowman fan.
The summer clothing the "general earth/ With greenness," just means that it's covering most or all of the earth.
The robin arrives to signal the end of winter, as smoke comes from a nearby cottage's thatched roof ("nigh thatch") and the sun thaws the earth.
Additionally, there's some mild S sound alliteration here in "all seasons shall be sweet." ("Shall" isn't really part of the alliteration though—Sh is a different sound from S.)
[…] whether the eave-drops fall Heard only in the trances of the blast, Or if the secret ministry of frost Shall hang them up in silent icicles, Quietly shining to the quiet Moon.
After mentioning spring and summer in the lines just before these, Coleridge finds value in the winter, too. It's got its place in the order of things and testifies to God in its own way.
The "eave-drops" are drops of water falling from the eave (the overhanging part) of a roof.
The frost is going to turn them into icicles.
Also, the "trances in the blast" are pauses in the wind. Coleridge calls them "trances" because it's like the wind is getting distracted and going still for that reason (going into a trance).
The icicles are "silent" because they stop the sound of the "eave-drops."
Continuing the pattern of reflection imagery, the frost—now, finally revealed as one of God's agents or ministers, performing secret work in the night (the "secret ministry" of the frost)—creates icicles which reflect moonlight back to the moon. Coleridge probably intends the frost to be doing work similar to the work that a poet should do: creating things that reflect divine truths and shining them back to God.
He also helps emphasize the silence of this communication with repetition—the "silent" icicles are shining "quietly" to the "quiet Moon."