Había amanecido
 

Hi,

It dawned, back then. Right? Is hay amanecido possible?

It had dawned.
This compound tense, called the pretérito pluscuamperfecto (the pluperfect, in English), uses a conjugated form of the auxiliary verb haber followed by a (past) participle. It is used when recounting an event.
No. 'Hay' means 'there is' or 'there are' (this is the existential form of haber). This will be followed by a noun (naming what is extant), not a participle.
Hay amaneceres que te dejan sin palabras. = There are sunrises that leave you speechless.

I agree with Rusty.
"Hay" is a particular conjugation of the verb "haber", which is only used when it means "there is/are".

The impersonal form you may be looking for is "ha amanecido".
- Estos días ha amanecido muy fresco el aire.
These days the air has been felt cool at dawn.
- Duérmete, todavía no ha amanecido.
Go back to sleep. It's not dawn yet.