A woman keeps saying the same thing to everyone: "It's just a bad day." On the surface it sounds like a casual brush-off, the kind of line we all use when we don't want to go into detail. The poem's last two lines change how we hear it. Nobody stops to think that she might be talking to herself as much as to them.
The speaker is an observer, and there's a quiet sadness in what they notice. The woman's words are meant to calm other people, but they work as a reminder to herself that things are fine, or will be soon. The repetition in "always" suggests this isn't a one-off. Maybe the bad days are piling up.
The mood is gentle and a little uneasy. Nothing dramatic happens, and that's the point: the pain is hidden behind an ordinary phrase, and the people around her miss it.