Arrays & Array Methods (map, filter, reduce)

const calendar = [
  [1, 2, 3, 4, 5, 6, 7],
  [8, 9, 10, 11, 12, 13, 14]
];
const dayIndex = calendar.findIndex(week => 
  week.includes(10)
);

The day of the week
The week containing a specific day
findIndex determines which week contains a day: 1) Searches through array of weeks, 2) Uses includes() to check each week for target day, 3) Returns index of first matching week, 4) Returns -1 if day not found, 5) Stops searching after first match, 6) More efficient than filter() for finding single item, 7) Common pattern in calendar applications, 8) Useful for data lookup operations.
The total number of weeks
The last day of the month