The speaker looks back on a sky that once seemed perfect: deep blue, full of white clouds that looked like a show, with birds everywhere and mild air. After each memory comes the same line, "I still remember exactly!" The repetition works like a chorus, and it makes the speaker sound eager, almost insistent, as if they are trying to hold the scene in place by saying it again and again.
The mood shifts in the last stanza. Time has "gone by so fast," and the sky is now gray. The refrain changes too: it begins with "But," which makes it sound less like a cheerful boast and more like a stubborn answer to loss. The speaker doesn't say why the sky has changed, and the poem doesn't explain it, so the gray could be literal weather or a sadder mood in the speaker's life. Either way, the feeling is gentle nostalgia mixed with a little defiance. The present is dull, but the memory of the blue stays bright.