Gray skies and a biting wind set the scene here, and the speaker's answer is a simple, childlike wish: a jar of sunshine. The idea is sweet and a little impossible, which is the point. Sunshine can't be bottled, so the wish carries a quiet longing for comfort that feels just out of reach.
The poem is built on repetition. "On days like this" comes back, and so does the jar, as if the speaker keeps returning to the same hope. Each time, the sunshine has a job to do. First it warms the skin and fills the speaker up, then it softens the wind from "sharp and cold" to "smooth and relaxing." That suggests the cold is more than weather. The speaker seems to want relief from feeling worn down or low, though the poem never says so outright. The short final lines, "oh I wish / for a jar of sunshine," sound almost like a sigh. The mood is gentle, wistful, and a bit lonely, but the image of warmth kept close keeps it tender rather than bleak.