I was amused by this post at Kenny Cargill’s Reading Notes in Russian Intellectual History, quoting Leibniz’s “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.
Actually, Russian среда [sredá], which also means ‘milieu’ and ‘medium,’ is (like those words) simply a semantic extension of the word for ‘middle’ (which is borrowed from Church Slavic; the native Russian form, also occasionally used, is середа [seredá]), Wednesday being in the middle of the week. I was briefly excited when Cargill checked this against his copy of Terence Wade’s Etymological Dictionary of Russian, with which I was not familiar — I thought it might be a useful alternative source of etymologies — but a bit of googling turned up this StackExchange question, which revealed that the Wade book only covers 1,500 words.