Use of Capitals in a Birthday Card

[icanneverremember]

Which would you write?

Dear X,

Happy Birthday!

Love Z

or...

Dear X,

happy birthday!

Love Z

Definitely capital H and B.

Capital H + B - why do you ask?

I would usually write capitals as well but I've been doubting myself and it's bugging me. Why the capitals - what rule does it follow?

I guess it's because it's an announcement - like a title, rather than part of a sentence

• but I'm not an inmate of Pedant's Corner

Happy birthday. Does this make me strange?

Capitals.Even if mid-sentence:

"Wishing you a very Happy Birthday!"

I personally wouldn't in mid-sentence, but would in a card.

But I'd be happy to get a card or a greeting whatever capitals were used.