I don’t have an exact date for you, but as someone with an interest in lesbianism and medievalism, I have actually seen references to strap-ons and sex toys a few times. We have evidence of strap-ons existing in the Middle Ages and early Renaissance from, of all places, court cases.

In the 13th century, a woman named Bertolina went to civic court in Bologna for sodomy with another woman. Among her various acts of sodomy, she was accused of possessing, “ using a certain mancipium (There is at least one other Bolognese case of similar date using this word to indicate a sexual instrument) with two silk testicles, conducting herself lustfully with women with this mancipium as men do with women. This is unutterable and horrible, in opprobrium of God and the world and against human nature.”

While the trial notes can be oblique in places, it is fairly evident that Bertolina was using a sexual device to perform sodomy, which in its description appears to have been a strap-on, though arguably, it may have only been an early dildo.

The trial of Katherina Hetzeldorfer in the 15th century raises the allegation of strap-on use a bit clearer: “She made an instrument with a red piece of leather, at the front filled with cotton, and a wooden stick stuck into it, and made a hole through the wooden stick, put a string through, and tied it round; and therewith she had her roguery with the two women….” The device she fashioned is tied to her body, thus creating the “strap” of “strap-on.”*

With this information, strap-ons at least existed as early as 1477, but based on earlier trial notes, may have existed as early as the 13th century.

Sources:

Lansing, Carol. 2005. “Donna con Donna? A 1295 Inquest into Female Sodomy” in Studies in Medieval and Renaissance History: Sexuality and Culture in Medieval and Renaissance Europe, Third Series vol. II: 109-122.

Puff, Helmut. 2000. "Female Sodomy: The Trial of Katherina Hetzeldorfer (1477)" in Journal of Medieval and Early Modern Studies: 30:1, 41-61.

*EDIT: I feel it important to note that Katherina Hetzeldorfer was tried for, among many things, wearing men’s clothing and having sex “as a man” though she was born female. While the court records call her a woman, Katherina’s own gender is not evident. Katherina might have been an early example of “butch” lesbianism, but may have also been an early example of a transgender man.