I mean, he's a high school student, and because of the number of teachers (and students) who were/are Jewish, the NYC school system gives us a lot of them off (Rosh Hashanah is usually the first day off we get after the school year starts after Labor Day, and we then get Yom Kippur off later in the month as well, Passover is usually rolled into our Spring Break as well). Same reason why we now get Lunar New Year off as well (actually, DeBlasio had to be shamed into it even though it was one of his campaign promises, but Cuomo was going to steal a March on him by doing it first, so he added it to the school holiday calendar as part of the oneupmanship war he had with our former Governor).
Peter might be aware of Jewish holidays just because they're days he gets off of school, like how I was aware of them growing up.