What does "the deep blue cup of the sky" mean in the poem "Laugh and be Merry"?

In poetry, the language is loaded with imagery. A cup
holds things, as does the sky: clouds, rain, lightning, the sun etc. Deep blue would be
describing the shade of blue the sky is reflecting from Earth's waters. Morning tends to
be a pale blue while later in the day, the shade would deepen. Poets want their readers
to see through the writer's eyes; so, they appeal to visual images. At times, they may
use a sound imagery like onomatopoeia or alliteration, but this is not the case in the
particular line you quoted.