Holst would certainly have known of astrology from his stepmother's Theosophical upbringing. But it was not until about 1912 that he took the subject further. In fact, [Clifford] Bax recalled that Holst told him apropos The Planets: ‘for two years I had the intention of composing that cycle.’ Since Holst began working on it in 1914 that would mean he started thinking about it in 1912[,] perhaps even earlier because Holst had a long gestation period for each major work.

As a rule I only study things that suggest music to me, that's why I worried at Sanskrit. Then recently the character of each planet suggested lots to me, and I have been studying astrology closely.