You look outside and it's sunny, you want to tell your spanish speaking friends that it's a lovely day, this is the way to do it:

Que dia mas bueno!
(it means 'it's a lovely day', literal translation is 'What a beautiful day')

you might also use the following phrases:
-Que tiempo mas bueno! (What a lovely weather!)
-Que sol mas bueno (What a sunny day!)
or in the day is not that good
-Que dia mas feo! (What an ugly day!)
Que tiempo mas malo! (What a horrible weather!)