The title and the first line “As imperceptibly as Grief” links the idea of grief to the passing of summer.
The tone is melancholic – “Twilight long begun” suggests that grief is overtaking her happiness.
The images of the end of a day/season such as “dusk”, “twilight” and “Summer lapsed away” suggest the passing of time and life.
“Summer made her light escape Into the Beautiful” suggests the passage of time is inevitable but will end in something more positive – possibly a release into heaven.
The tone of the final words is more hopeful than the rest of the poem.