The poem opens by declaring that it is “three days before Armistice Sunday.” This seems to be a combination of two public holidays in England honoring soldiers who died in World War I: Armistice Day (which actually falls on a Monday), and Remembrance Sunday. Right from the poem's opening line, then, war hovers in the background.
The poppy is also part of these holidays: since 1921, people have used paper poppies—also called “remembrance poppies”—to honor fallen soldiers, either by leaving poppies on soldiers graves and wearing them on their clothes. The flower symbolizes the war—and, more broadly, the death and grief that war creates.
Meanwhile, pinning a flower to someone's lapel—the folded front part of a suit coat or jacket—is a tender act that a parent might do for a child on a big day (or, perhaps, that a person might do for a lover—though, as the poem continues, it'll become clear that this is not a romantic relationship). This clues the reader in as to who this poem is about, but it is not clear yet exactly how old this child is—whether this child is an adult going off to war, or simply a younger child going off to school. Part of the confusion comes from the word "blazer," which could refer to a child's school uniform or to a military uniform.
Both interpretations are valid, and the speaker uses references to school as an extended metaphor: sending a child to school is like sending a child off to war. It is a much smaller version of the same thing, and produces similar anxiety and grief in parents who are no longer able to shield their children from the dangers of the world. So even if the child's "blazer" is part of a school uniform, it is also a metaphor for a soldier's uniform. (Of course, sending a child to school comes with the assumption that a child will return home; parents sending a child to war do not have such certainty.)
The speaker also takes an unusual perspective on war. Instead of focusing on the bravery of the soldiers or the glory they win in battle, she is concerned with the grief of those who stay behind—especially the parents of soldiers.
In the poem’s opening stanza, the speaker provides a couple of hints that help the reader get a handle on the poem’s extended metaphor. Note, for instance, the way the speaker describes the paper poppy. It’s a tightly folded paper flower, with petals overlapping each other—potentially something very beautiful. But the speaker describes it almost as a wound, using a series of metaphors. Its folds are like “spasms,” involuntary and painful contractions of the muscles. It “disrupt[s] a blockade / of yellow bias binding.” Literally, the speaker is just saying that it covers up the yellow border of her child’s blazer. But the word “blockade” is a military term: it often describes a flotilla of ships blocking off an enemy port, so they can’t bring supplies or troops in. The metaphor makes it sound like the paper flower has broken through the lines, causing death and destruction.
An alliterative /p/ sound also runs through the passage, linking together “poppies,” “placed,” and “pinned.” The alliteration suggests that “pinn[ing]” a poppy on the child’s jacket is like placing one on a grave: the speaker feels like this is an act of memorial and grief for the child. In this sense, the alliteration reinforces the extended metaphor: the child is not just heading to school but, metaphorically, to war.
The first six lines of the poem establish its formal pattern. It’s written in free verse: it doesn’t have any meter or rhyme. The tone is direct and conversational. But the poem still betrays the speaker’s anxiety and grief. Note, for instance, how heavily enjambed these opening lines are. Despite the matter-of-fact tone, the speaker is anxious and rushing, the poem spilling urgently down the page.