Since "happy birthday" is not a proper noun, it should not be capitalized in the middle of a sentence.

Since "happy birthday" is not a proper noun, it should not be capitalized in the middle of a sentence.

I just wanted to wish you a happy birthday.

I just wanted to wish you a Happy Birthday.

Another example:

Mary didn’t have a very happy birthday because she lost her job.

When to Capitalize "Happy Birthday"

If "happy birthday" begins a sentence, "happy" is always capitalized.

Happy birthday, Patricia!

If "happy birthday" is part of a headline or title, capitalize both words.

Ten Creative Ways to Say Happy Birthday.

When using "happy birthday" as part of a greetings card or message to say your birthday wishes, you can capitalize both words or the entire message for emphasis.

Hoping That You Have a Happy Birthday

If "happy" is the first word in a complete quotation, capitalize it, even in the middle of a sentence.

Robert said, "Happy birthday, Lisa."

Notice that when addressing someone directly (e.g., in dialogue), we enclose names between commas.

Happy birthday, Jennifer.

We'll add here that when it's someone's birthday, we just say "happy birthday", and not "congratulations".