… or thereabouts; the canonical date is “the early spring”.
April 13, 1890: Violet Hunter sat in the window seat for the second time [COPP]
“Two days later this same performance was gone through under exactly similar circumstances. Again I changed my dress, again I sat in the window, and again I laughed very heartily at the funny stories of which my employer had an immense repertoire, and which he told inimitably. Then he handed me a yellow-backed novel, and, moving my chair a little sideways, that my own shadow might not fall upon the page, he begged me to read aloud to him. I read for about ten minutes, beginning in the heart of a chapter, and then suddenly, in the middle of a sentence, he ordered me to cease and change my dress.”
A Day by Day Chronology of Sherlock Holmes according to Ziesler and Christ by William S Dorn DWNP, BSI.