*Je suis samedi*, *tu es samedi*, *vous êtes samedi* or *ils sont samedi* would be, for native speakers, as puzzling as *nous sommes samedi* is for a non native person.
A reason is probably the lack of preposition that suggests there is some sort of identity between *nous* and *samedi*.
A preposition removes this issue in *nous sommes en France*, *nous sommes en 2020*, *nous sommes en mai*, *nous sommes au printemps* just like does *in* in English (we are in France/in 2020/in May, in Spring). There is no preposition suitable with a day of week, *nous sommes en lundi* is no more acceptable t…