Apparently with no surprise To any happy Flower (1-2)
So these flowers aren't surprised when they die; in fact, they seem happy despite the fact that they know death is creeping toward them. That's usually not true with humans, right? We think it's safe to say that most people agree that death isn't anything to smile about. Is it possible that the poem is holding these flowers up as an example of how we ought to die? Maybe the poem is saying, "The flowers aren't scared. Why should you be?"
The Frost beheads it at its play— In accidental power— The blonde Assassin passes on— (3-5)
The frost is a big, whopping symbol of death in this poem. It's the cold-blooded killer that sneaks in and does some flower killing. This is far from the only poem that uses icy imagery to symbolize death in general. This poem isn't just about flowers dying, it's about how all things die. Thanks, Emily; we're going to go cry in a corner now.
The Sun proceeds unmoved To measure off another Day (6-7)
Question: What will eventually kill every living thing? Answer: Time. Here, the image of the uncaring sun "measure[ing] off another Day" symbolizes the inevitable passage of time, which is dragging us all toward you know what. Ultimately, though, the poem's take on this is kind of ambiguous. Is it saying this is unfair? Or is it just pointing out that it is the way it is?