fine day; clear day
Refers to a day with good weather, often used in contrast to rainy or cloudy days.
Tomorrow looks like it will be a fine day.
I like taking walks in the park on clear days.
formal occasion; special day
Used for ceremonies, celebrations, or important life events such as weddings and graduations.
Many people wear kimono on formal occasions.
Graduation day is a special day in one's life.
Compound of 晴れ (clear weather, formal) + の (possessive particle) + 日 (day). The figurative sense derives from the idea of a 'clear, bright day' being associated with special, auspicious occasions.