The first time I read this poem I imagined the “simple days” as a group of children holding hands as they play or cross a road. I suppose what made me think of that image was how when we are young time seems to pass much more slowly which she describes here as our own “mortality” being just a “trifle”, as if we don’t care that we will one day day because today time seems to pass so slowly that the fateful day will never come. Emily is saying that each day carries off with it just a “trifle” of mortality each day and as the days turn into seasons then we become more aware of how much time has passed and how much closer to death we are. But in the moment we do “venerate the simple days” because, like children, we live in the moment and don’t notice how time is slipping past until we’re much older and then we’re racing just to keep up.
Unlike the first stanza which deals with the passage of time, the second stanza deals with our place in the world, specifically how though the “days” may be “simple”, our small “Acorn”-like existence can grow into a forest “For the upper air”. There is a majesty and a dreamlike quality to the image of the tops of trees rising up out of acorns and into the sky above, but the “upper air” could also be referring to heaven and that our life is just an “Acorn” and that we won’t really hatch (as from an “Egg”) until we die. In fact the final image, as beautiful as it is, is also sad in how it seems a little futile, that our lives will become little more than an audience in the “upper air”.
In fact the whole poem is quite sad as it deals with the slow passage of time leading to the eventuality of death and how after death we are bound only for “the upper air”. She seems to be combining both the beauty of being alive with the uncertainty of what’s to come.