On April 11th…

… or thereabouts, the canonical date is in “the early spring”.

April 11, 1890: Violet Hunter in window seat for the first time. [COPP]

Friedrich illustration of Violet Hunter sitting in front of the window.
Illustration by Josef Friedrich

“A chair had been placed close to the central window, with its back turned towards it. In this I was asked to sit, and then Mr. Rucastle, walking up and down on the other side of the room, began to tell me a series of the funniest stories that I have ever listened to. You cannot imagine how comical he was, and I laughed until I was quite weary. Mrs. Rucastle, however, who has evidently no sense of humor, never so much as smiled, but sat with her hands in her lap, and a sad, anxious look upon her face. After an hour or so, Mr. Rucastle suddenly remarked that it was time to commence the duties of the day, and that I might change my dress, and go to little Edward in the nursery.”

Source
A Day by Day Chronology of Sherlock Holmes according to Ziesler and Christ by William S Dorn DWNP, BSI.