The Clue When many a party ends (Hint: Latest eclipse)
The Answer DAY
The answer "DAY" fits the clue because many parties end at the end of the day. The hint "Latest eclipse" refers to how eclipses often occur during the day, but they can also create a moment of darkness. When the eclipse is over, it marks the return of daylight. Thus, a party that ends after the eclipse would indeed end during the day.