Few words carry a lot here. The title, "Vigil," suggests a night spent watching over someone, and the poem seems to place us beside a person who is ill or fading. The speaker never says so outright, which is part of the poem's quiet power; judging by the imagery, the "her" in the second stanza is someone close, and the speaker is watching her weaken.
The opening lines make the days feel sudden and unavoidable, "as immediate as doors," as if each one swings open without warning. Fear is so big that it "would not fit / into our bed," so even the place meant for rest and closeness can't contain it. The image of her face as "a dying aster" is gentle and sad at once: a flower, delicate and star-shaped, beginning to wilt. The closing question, "how to hold it," works two ways, since the speaker wonders how to hold a face, a fear, or a moment slipping away. The mood is hushed, tender, and helpless.