Based on the true history of our era, the book "Good Omens", nice also means "scrupulously exact,"
The dictionary disagrees but what do they know. Their fake origin was this:
Word History Five hundred years ago, when nice was first used in English, it meant "foolish or stupid." This is not as surprising as it may seem, since it came through early French from the Latin nescius, meaning "ignorant." By the 16th century, the sense of being "very particular" or "finicky" had developed. In the 19th century, nice came to mean "pleasant or agreeable" and then "respectable," a sense quite unlike its original meaning.
So you are being wished a scrupulously exact day and not a foolish, stupid, ignorant or finicky day.