The portrait, Whistler's Mother, may date from 1871. It has been dated stylistically, and from the date when Mrs Whistler was known to be posing to Whistler. In the summer of 1871 Whistler asked her to pose for a painted portrait, and she described the early sittings to her sister:
'I was surprised when the next day he said to me "Mother I want you to stand for me! it is what I have long intended & desired to do, to take your Portrait." I was not as well then as I am now, but never depress Jemie by complaints, so I stood bravely, two or three days whenever he was in the mood for studying me. his pictures are studies & I so interested stood as a statue! but realized it to be too great an effort so my dear patient Artist (for he is greatly patient as he is never wearying in his perseverance[)] concluding to paint me sitting perfectly at my ease' 1
1: A. M. Whistler to C. J. Palmer, 3-4 November 1871, GUW #10071.
These sittings eventually resulted in the famous oil painting, Arrangement in Grey and Black: Portrait of the Painter's Mother [y101]. However, this is the only reference to Anna Whistler literally standing for a portrait, and it may be that he made a quick drypoint sketch of her, before turning to a seated pose.
However, it is also possible that it could date from the following years, 1871-1872, when Whistler was experimenting with drypoint portraits and variations in printing effects.