Q:

"It was an April" vs "It was in April"

"It was an April afternoon. And the warm breezes of approaching summer
  had sent everyone, including Grandfather, indoors".

The above sentences are from the story 'Adventures in a Banyan Tree' by the Indian English writer Ruskin Bond. (The context of the story is a school boy's summer vacation excitements and experiences, which he narrates after the vacation. The summer vacation falls in April and May)
Based on these sentences, a question for editing came in an examination. But in the question, they used only "April" instead of "April afternoon" as shown below:

"It was (a) at April. The warm breeze of the approaching summer (b)
  have sent everyone indoors including grandfather".

Now my question is about the answer to (a): can it be an as in the actual sentence in the story?
Or,
Should it be in, the preposition before the name of a month?
[This question I asked on ELL a little while ago and I got an answer there. But the answer does not clear my doubt whether I can use in or not, as in It was in April.]
If the sentence were something like It was in April (a strange thing happened.), wouldn't it grammatically be an acceptable sentence?

A:

I think all options are permissible but not equally desirable.  It depends on personal preference and style.  The most common usage would be "It was April".  This contrasts April with other months i.e. it wasn't May.  "It was an April" suggests maybe that the writer can't remember which April it was.  Doesn't necessarily seem wrong to me but contains an element of redundancy.  "It was in April" emphasises that the event happened in April but would often be followed by "that" as in "It was in April that this event happened".
I would choose "It was April" as the answer.

