I saw in a medrash - but can’t recall where - that the brothers sold Yosef on the tenth of Teveis.
Does any which medrash I could find this in?
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As far as I can tell, you didn't see it in a traditional "Midrash", since I've researched it pretty seriously and haven't come up with anything.
However, in a fairly recent "Midrash", a book of Derashot by Rabbi Shlomo Fischer, "Derashot Beit Yishai" (page 242, in Siman #30, which is his Derashah for the 10th of Tevet; Otzar Link), he makes this suggestion. He himself notes that it hasn't been said before:
ואילו מכירת יוסף לא פירש באיזה יום היה ולולי דמסתפינא אמינא שהיה ביום עשרה בטבת
He develops this idea with a couple of hints and other connection, as have various modern authors (a Google search in Hebrew can easily confirm this). Also important to consider is that the Parshiyot detailing the sale of Yoseph are read just before the 10th of Tevet.
Some posit that Yosef's sale by his brothers, occurred on Yom Kippur.
See Hebrew discussion here: https://simla.otzar.org/viewtopic.php?t=16134