Cadmus married Harmonia. Their wedding was the first between mortals that was ever attended by Olympians, whose favor ensured Cadmus a long and successful reign.
Was Cadmus the only mortal whose wedding was attended by Olympians?
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Cadmus may have been the first mortal (Harmonia was immortal) to be married while Olympians watched, but he was not the only one.
Peleus and Thetis were married on Mount Pelion. Thetis was, according to some stories, immortal, but Peleus himself was mortal.
While on mount Pelion, Peleus married the Nereid Thetis, by whom he became the father of Achilles, though some regarded this Thetis as different from the marine divinity, and called her a daughter of Cheiron. (Apollon. Rhod. i. 558; comp. THETIS.) The gods took part in the marriage solemnity, and Cheiron presented Peleus with a lance (Hom. Il. xvi. 143, xxiv. 61, &c., which, however, according to Pindar, Nem. iii. 56, Peleus made for himself), Poseidon with the immortal horses, Balius and Xanthus, and the other gods with arms. (Apollod. iii. 13. § 5; Horn. Il. xvi. 381, xvii. 443, xviii. 84.)
Eris was not invited, however, and began the Trojan War.
Additionally, Hymenaios was supposed to attend every wedding, being the wedding god himself.