Filtering a Date attribute for a CoreData Entity can be very confusing. The documentation didn't help me to solve this, but ChatGPT got me into the right track. Here is my solution:
 /// Creates a predicate to fetch a `key` attribute in a certain `date`
 func makeFetchRequestPredicate(for key: String, on date: Date) -> NSPredicate {
    let calendar = Calendar.current
    let components = calendar.dateComponents([.year, .month, .day], from: date)
    let currentDay: CVarArg = calendar.date(from: components)! as CVarArg
 let nextDay: CVarArg = calendar.date(byAdding: .day, value: 1, to: calendar.date(from: components)!)! as CVarArg