'go to bed at midnight' vs. 'on midnight'

[Stone]

English Grammar Tests, Elementary Level

ESL/EFL Test #96 "prepositions of time: for, ago, since, on, at", question 2

I like to go to bed ......... midnight on Saturdays.

(a) since
(b) on
(c) during
(d) at

English Grammar Tests, Elementary Level

ESL/EFL Test #96 "prepositions of time: for, ago, since, on, at", answer 2

I like to go to bed at midnight on Saturdays.

Correct answer: (d) at

Your answer was: incorrect
I like to go to bed on midnight on Saturdays.
_________________________

Dear Sir

Why not "on" here

Thanks a lot in advance

Stone

[Yankee]

Hi Stone

'Midnight' is a specific time and therefore you need 'at'.

Use 'on' for a specific day or date.

Amy