The Nuttall Encyclopedia
Days of peace, happiness, and prosperity, properly the seven days before and the seven after the winter Solstice, days of quiet, during which the halcyon, or kingfisher, is fabled to be breeding.
Wood, James, ed. Entry for 'Halcyon Days'. The Nuttall Encyclopedia. https://www.studylight.org/encyclopedias/eng/nut/h/halcyon-days.html. Frederick Warne & Co Ltd. London. 1900.