"How beautiful you are" can be expressed in a few different ways. 1. 얼마나 아름다운지 몰라요! / 얼마나 아름다운지요! / 넌 얼마나 아름다운지 몰라! (between friends) 2. 너무나 아름다우세요! / 너무나 아름다워요!/ 너 너무 아름답게 보여! #1 is quite similar to "How beautiful ..." in English, while #2 is like "You are so beautiful!". In daily conversation #2 is more commonly used. #1 sounds more formal and literary. There doesn't seem to be an expression that directly corresponds to "What a day". You can say what is extraordinary about the day using 얼마나 or 너무나, or use some other expressions. - 얼마나 경이적인 날이었나! What an awesome day it was! - 너무나 놀라운 날이었다. It was a truly wonderful day. - (참으로) 대단한 날이었다! It was (really) an extraordinary/remarkable day.

"~구나" or "~군요" is used for exclamatory sentence in Korean For example, "(정말) 아름답구나!" or "(정말) 아름다운시군요!" which means "how beautiful you are" (정말 means really) However, I've hardly speak and listen exclamatory sentence in Korean We don't use for greeting :(