A friend of mine used to greet me that way. He was long- and fully lived, having been through wars and children and loss and a multitude of jobs. He never failed to see some good or potential for good in his every day.
He also said it in a particular way, which made the phrase worth the hearing every time I saw him. He started with an emphatic, "It's", then dropped off in volume and tone, building back up to the same emphasis at "day!".
As it scrolls through my mind's ear even now I can remember his face and voice and demeanor of believing the words he'd just uttered fully. It was quite infectious and I adopted the phrase in his unique voice and say it quite regularly.
So, next time you're greeted, I challenge you to reply with: It's another beautiful day!