As a reminder, they're attending a
Catholic school, not just Christian. And there are usually multiple Mass services offered over the weekend that can fulfill the Sunday worship obligation, which can include Saturday evening as well as services throughout the day on Sunday. They're not limited to the morning. And as an example, here's a truncated listing of Mass times for a Catholic Church in Tokyo, which does include Saturday evening (the 6PM vigil Mass listed first).
Besides, Mass services typically last about an hour (exceptions include special days of obligation like Easter and Christmas), and Naomi was planning to meet with his cousin at 2 in the afternoon, which is enough time for even a noontime Mass service.