On October 31, 2020 a blue moon and red Mars will dominate night skies. This Halloween boasts the second full moon in a calendar month and closest approach of Mars to Earth in two years. Despite “once in a blue moon” folklore, blue moons aren’t that rare. The Moon completes a phase every 29.5 days, a hair shy of our calendar month. As such, every two to three years a 13th full moon occurs. We call it a blue moon. There won’t be another blue moon until August 31, 2023. Not another full moon on Halloween until October 31, 2039.
Astronomers regard Halloween as one of four annual cross quarter days. Cross quarter days fall midway between an equinox and solstice. Halloween being the midway point between the fall equinox and winter solstice. Halloween joins Groundhog Day (February 2), May Day (May 1) and Lammas (August 1) to complete cross quarter day’s roster. Cross quarter day blue moons are significantly rarer than run of the mill blue moons.
Below – a bright Mars teaser from Abigail Atlenza at https://earthsky.org/