Okay, not literally—it's just the day Young Ju's new baby brother is coming home from the hospital.
Gomo's at the house taking care of Young Ju, but she isn't really taking care of Young Ju; she's really just getting the house ready for the new baby.
She does, however, manage to tell Young Ju what a Mongolian spot is—a birthmark that apparently every Korean baby has because Koreans used to come from Mongolia.
Then he comes home, and everyone (except Young Ju) goes totally gaga over the little baby; he's even got the adults forecasting that he's going to be president one day.
Young Ju feels left out and calls out that she could be president one day, but Apa just laughs at her and says she can't because she's a girl.
(Yeah—Apa's not exactly up for any Father of the Year awards.)
All Young Ju gets is a mini-lecture from Gomo about how she's now an older sister, so she's responsible for helping to take care of the little runt. Oh—and that she needs to be a good housekeeper too. What a future.
Young Ju gets it into her head to hunt for the baby's Mongolian spot because maybe, if he doesn't have one, then he can't really be a Park. You know—perfectly logical stuff for a young girl to think of.
Anyway, the baby has a Mongolian spot, so no dice: he's a Park.
Young Ju notices how Apa's hands are soft and clean—which isn't how his hands usually are.
In fact, Apa just has a totally different, better way of treating the new baby (different from, she's guessing, the way he reacted to her birth).
The adults all toast to the new baby, Park Joon Ho, while Young Ju thinks to herself that she's "not a baby anymore."