no ON

some people say that ON is optional/unnecessary
https://www.usingenglish.com/forum/threads/105351-last-Friday-or-on-last-Friday
https://forum.wordreference.com/threads/last-friday-or-on-the-last-friday.1741639/

Are they mistaken?

Absolutely! https://books.google.com/ngrams/graph?content=saw+%2A+on+last+Saturday%2Csaw+%2A+last+Saturday&year_start=1800&year_end=2019&corpus=26&smoothing=3&direct_url=t2%3B%2Csaw%20%2A%20last%20Saturday%3B%2Cc0%3B%2Cs0%3B%3Bsaw%20him%20last%20Saturday%3B%2Cc0%3B%3Bsaw%20you%20last%20Saturday%3B%2Cc0%3B%3Bsaw%20them%20last%20Saturday%3B%2Cc0%3B%3Bsaw%20it%20last%20Saturday%3B%2Cc0%3B%3Bsaw%20her%20last%20Saturday%3B%2Cc0%3B%3Bsaw%20me%20last%20Saturday%3B%2Cc0

https://ell.stackexchange.com/questions/96390/why-do-we-not-use-a-preposition-like-on-in-i-met-him-last-friday

Well, what if it's not SAW?
https://www.google.com/search?q=%22on%20last%20saturday%22&tbm=bks&tbs=cdr:1,cd_min:1945,cd_max:2019&lr=lang_en

I don’t know what to tell you except that it sounds highly idiotic and I can’t explain why it’s in those books unless they’re quoting the actual speech of an uneducated character.

OK

The only exception would be if ON were part of a preceding verb phrase: For the picnic, we were planning on last Friday, but it rained all day.”

Здесь я перевел бы "планировали на прошлую пятницу"

👍🏼