It's the eve of Michaelmas, the great feast of St Michael and All Angels - an invocation of the powers of light as the year turns towards the darker days of autumn. Here's a thread on Michaelmas daisies, geese, moons, and other traditional namesakes of the angels' feast.
Michaelmas, on 29 September, is not only an important festival in the church year but in England also historically the autumn quarter-day. It marked a moment of transition: the shift from the harvest season into later autumn, the start of new contracts and new school terms.