Learn English – Used to have or used to had

I don't think its a problem of being "grammatically correct" or not. Its more about what is the commonly accepted naming convention.

Labor Day, Arbor Day, Child Health Day

Follows the formula of [noun] Day (which is what Friendship Day would fall under)

Native Americans' Day, National Library Workers' Day, Parents' Day

Days for groups of people seem to have apostrophes.

Martin Luther King Day, Prince Jonah Kuhio Kalanianaole Day, Harvey Milk Day

Days for individual people don't have apostrophes unless..

Valentine's Day, St. Patrick's Day, St. David's Day

It seems like religious(christian) holidays involving a specific person utilize apostrophes.

They can both be correct. It depends on the context. Here is how:

I had been listening to his songs since childhood

• ..when he released the album "The Wind", which quickly became my favorite.

• ..when he changed his style, which was hard for me to understand.

• ..when I met him, and I didn't know what to say.

"Had been" is used to express the past tense when something else happens. With "had been", since needs to have a finish, or end-point. Since you start the sentence with "I", you need to finish the thought with how it affects you.

I have been listening to his songs since childhood

• and I think he is a genius.
• and he is still my favorite.
• but I no longer think he is the best.
• although I prefer other artists now.
• yet I still cannot understand them.

"Have been" is also used to express the past tense, but where the context is now. "Since" does not require closure if the conclusion of "since" is "now, at this moment". But if you add another part to the sentence, like the above examples, so you would finish the sentence with your position now.

Just for completeness, there is another important version of the past tense, the "simple past":

• I listened to his songs in my childhood.
• I listened to his songs as a child.

This does not require any followup. The choice depends on the context where you are placing the idea.