Three short lines, and they hold a surprising amount of tension. The title tells us we are at a nativity, a birth, and the image we expect is a calm, glowing night with a guiding star. Instead, the poem gives us "a rose of blood flying in the heavens." The sky is not quietly shining; something red and living seems to be moving across it.
A rose is a symbol of beauty and tenderness, while blood suggests pain, violence, and sacrifice. Putting them together makes one image that is both lovely and unsettling. It may hint at the physical reality of birth, which involves blood as well as joy. It may also point toward suffering to come, if we read the title as a reference to the Christian story of Christ's birth.
The speaker doesn't say how they feel. They simply observe, and that restraint makes the mood hushed, awed, and a little ominous. "Bright night" sets wonder against dread, and the poem leaves us caught between them.