In the following sentence
I have not seen him since Wednesday last.
How is last used?
I think it should be
I have not seen him since last Wednesday.
The question was answered in the comments:
"I have not seen him since Wednesday last" is grammatical but a bit poetical. The second sentence would usually be used. - Weather Vane
It's old-fashioned or poetic.
if someone used "Wednesday last" in conversation thinking that was a normal way to say "last Wednesday," they'd still be very wrong. – the-baby-is-you