The poem is three short lines, and the whole feeling lives in the gap between the first line and the next. The first line is just an object: a son's old kite. The word "old" matters. It suggests the boy has outgrown it, or time has moved on, and the kite is now something kept rather than used. The poem never says what the speaker feels about that, which is part of its pull.
Then the speaker puts on a velvet coat and heads to town. Velvet is soft, dressy, a little formal, so the move from kite to coat feels like a step from play into ordinary adult life. The title, "First day of spring," adds a note of renewal, and kites belong to windy spring days, so the old toy sits oddly against the season's fresh start.
The mood is gentle and wistful, a small tug of memory carried through a normal errand. Nothing dramatic happens, and the poem trusts us to feel the quiet passing of time.