Can we say 'at saturday night'?

[Foka]

English Language Tests, Intermediate level

ESL/EFL Test #206 "Prepositions of Time and Date (4)", question 7

......... Wednesday night, I'm going to take the staff out for dinner.

(a) About
(b) In
(c) On

English Language Tests, Intermediate level

ESL/EFL Test #206 "Prepositions of Time and Date (4)", answer 7

On Wednesday night, I'm going to take the staff out for dinner.

Correct answer: (c) On

Your answer was: ?
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can we say 'at saturday night' ?
if no, why ?
Foka

[Mister Micawber]

.
No. We use 'at' for time ('at 8 o'clock', 'at noon') and in the idiomatic 'at night'. If you put 'Saturday' into the phrase, it is no longer idiomatic, but takes 'on', which we use for days: 'on Saturday', 'on June 1st', etc.
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Native English teacher at Mister Micawber's