This is the 8126th most frequent Swedish word.
afton
Evening.
"Aftonen" is used here in the definite form to describe the specific evening they are referring to.
Om kvällen möttes de i parken för att samtala och njuta av aftonen.
In the evening, they met in the park to converse and enjoy the evening.
"Julafton" incorporates "afton" as part of a compound word indicating a specific celebratory evening, in this case, Christmas Eve.
På julafton samlas familjen för att fira högtiden med mat och presenter.
On Christmas Eve, the family gathers to celebrate the holiday with food and gifts.
"Aftonens" is used in the genitive form to denote relation to the start of the evening.