Leibniz proposes an odd etymology for the Russian word for Wednesday (sreda), as though the word derived from the name of Zarathustra:
His [Zarathustra’s] great learning caused the Orientals to compare him with the Mercury or Hermes of the Egyptians and Greeks; just as the northern peoples compared their Wodan or Odin to this same Mercury. That is why Mercredi (Wednesday), or the day of Mercury, was called Wodansdag by the northern peoples, but day of Zerdust by the Asiatics, since it is named Zarschamba or Dsearschambe by the Turks and the Persians, Zerda by the Hungarians from the north-east, and Sreda by the Slavs from the heart of Great Russia, as far as the Wends of the Luneburg region, the Slavs having learnt the name also from the Orientals.
Yet Terence Wade (in his Etymological Dictionary of Russian, p. 208) notes that the word is related to other terms that emphasize the meaning of "middle" (like serdtse, seredina, and srednii). Hence, like the German Mittwoch, it just means the middle of the week.
Though Leibniz may have known some Russian from being the confidant of Peter the Great, he lived long before the era of modern linguistics and thus knowledge of sound changes. I would thus be willing to believe Wade. It couldn't be true that both etymologies are correct?