It is difficult to establish the point at which Holst began in earnest to consider a large suite based on astrological influences. There are no early sketches or studies for such a suite; and, in fact, the piece seems to have sprung virtually full grown out of his head during the summer vacation of 1914. There is mention of the sketch for Mars having been completed during the summer of 1914, soon after the composer had moved out of London to Thaxted, but when he began it is unknown. Clifford Bax quotes Holst as saying that he had had the intention of composing the work for two years, during which time “it seemed of itself more and more definitely to be taking form.” There is no way of knowing whether this referred to the period of composition (1914–16) or to the period before his first sketches. The latter possibility seems to be ruled out by Bax's earlier belief that Holst had not known much about astrology before their 1913 holiday. On the other hand, neither Bax's claim nor Holst's reminiscence can be taken absolutely on faith, for neither was meant as an attempt to fix the precise date of the work's inception.

Another possibility, as Michael Short suggests, is that Holst had begun considering a suite for large orchestra in 1912, after hearing Schoenberg's Five Pieces for Orchestra. That suite had been given a second performance in January of 1914, and, with the added impetus of Holst's growing interest in astrology, his ideas which had been incubating in the abstract for nearly two years may have been brought to life.