What happens only in the middle of each month, in all of the seasons, except summer and happens only in the night, never in the day?

Why this works

This riddle cleverly plays with the concept of letters and their positions in words. The letter 'n' is found in the middle of the word "month," which occurs in every season, but it's not present in the word "summer." Additionally, 'n' appears in the word "night," but not in "day." This combination of linguistic elements makes the answer both surprising and satisfying!