Occult Day takes place on November 18, 2018. The occult is "knowledge of the hidden". In common English usage, occult refers to "knowledge of the paranormal", as opposed to "knowledge of the measurable", usually referred to as science.
The term is sometimes taken to mean knowledge that "is meant only for certain people" or that "must be kept hidden", but for most practising occultists it is simply the study of a deeper spiritual reality that extends beyond pure reason and the physical sciences. The terms esoteric and arcane have very similar meanings, and in most contexts the three terms are interchangeable.
Occult also describes a number of magical organizations or orders, the teachings and practices taught by them, and to a large body of current and historical literature and spiritual philosophy related to this subject. (With material from: Wikipedia)