for memories
Aki 2014 Day 13 Kakuryu lost against Ichinojo and Hakuho lost against Goeido - Harumafuji was absent
Nagoya 2009 Day 11 Asashoryu lost against Kaio and Hakuho lost against Kotomitsuki - nice day for my all-time favourites rikishis Kaio-(Koto)Mitsuki !
and many time before of course
but three Yokozunas ?
last time Aki 1999 Day 2 Wakanohana lost against Kotonishiki, Takanohana lost against Musoyama and Musashimaru lost against Tochiazuma - Akebono wins against Kyokushuzan.... So It's not the grand slam lol ;)
Last complete sweep
Haru 1984 Day 5
Kitanoumi lost against Hoshi, Chiyonofuji lost against Daijuyama and Takanosato lost against Onokuni !
In the modern era, with four Yokozunas in the Banzuke, never the four Yokozunas have lost the same day.