Había amanecido

[Tyrn]

Hi,

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

[Rusty]

Originally Posted by Tyrn

It dawned, back then. Right?

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.

Originally Posted by Tyrn

Is hay amanecido possible?

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.

[AngelicaDeAlquezar]

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.