(From Bright Hub Education)
The prepositional phrases can perform six
nominal functions. Nominal functions are grammatical functions
prototypically performed by nouns, noun phrases, and noun clauses. The
six nominal functions prepositional phrases are:
Prepositional phrases of location and time most often perform nominal
Behind the shed needs mowing.
Before ten is a bad time to call me.
Under the refrigerator is disgusting!
In the light of this explanation, I believe, the OP's following sentences are grammatically acceptable.
"On Friday is a concert at Trafalgar Square."
"On the table is a book".
The following example sentence (from The Internet Grammar of English) shows a prepositional phrase of time functioning as the subject of the sentence.
Prepositional Phrases functioning as Subject
Less commonly, the Subject may be realised by a prepositional phrase:
After nine is a good time to ring
Prepositional phrases as Subject typically refer to time or to space.