The poem was written in the year without a summer (https://en.wikipedia.org/wiki/Year_Without_a_Summer) when Byron and the Shelleys took refuge at Villa Diodati (that's where Mary Shelley got the idea for Frankenstein). What happened was a worldwide volcanic winter caused by eruption of Mount Tambora a year earlier. Neither of the two collected editions of Byron's works I own mentions this fact -- literary scholars are often strangely oblivious of the real physical world.