Popular Astronomy, Vol. 24, p.639.
Report on Mars, No. 17
By William H. Pickering
EDITOR'S NOTE: Although in his 1895 book Mars, Percival Lowell used dates to refer to the time of year on Mars, his system was not one that someone living on Mars would have found useful. He merely used the familiar convention of the Gregorian calendar and stretched it to fit the orbit of Mars. Thus, there were twelve months, all with the usual names, and thirty days had September... et cetera. Really, this was nothing more than a disguised form of the solar longitude system that astronomers and planetary scientists use to this day, but Lowell was writing for a general readership that had never heard of solar longitude, so whereas to fellow astronomers he would have referred to LS = 90° for the northern hemishperic solstice on Mars, he chose instead to express it as June 21. The drawback of such a system for anyone living on Mars is that June 22 comes not quite two full sols later, because Mars takes 1.88 Earth years to complete one circuit around the sun, so what would a citizen of Mars call the sol between June 21 and June 22... June 21 and 1/2? This example is untidy enough, but because the Martian year is not quite twice as long as an Earth year, some dates in Lowell"s system, say for example December 1, would be followed by December 2, with no "half date" between. One can see that Lowell's system was a convenient shorthand for him to use to communicate to his readership the concept of seasons on Mars without getting the pace of his narrative bogged down in technical details. It should be remembered that Lowell was a writer first and an astronomer second, which is why his books were a culture phenomenon in his time and long afterward, inspiring H. G. Wells, Edgar Rice Burroughs, Robert A. Heinlein, Isaac Asimov, Arthur C. Clarke, Ray Bradbury, Leigh Brackett, Lin Carter, and others from the 1890s into the 1980s.
The first known practical Martian calendar appears to have been devised by two of Lowell's proteges, William H. Pickering and Andrew E. Douglass. The original text in which they explain their calendar has not been found; however, the Italian astronomer Mentore Maggini mentions their calendar in his 1939 book, I pianeta Marte. Below is a table from Pickering's 1916 paper in Popular Astronomy. The column "1916" gives Gregorian dates, the column "☉" gives corresponding values for the solar longitude of Mars (LS), and the column "M.D." give the corresponding Mars dates. This is the first known use of a Martian calendar in a scientific paper.