Write a method called dayorWeek that accepts an integer as a parameter representing what day of the year it is out of 365 with 1 being the first day of the year and 365 being the last (for example: 142)

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Write a method called dayorWeek that accepts an integer as a parameter representing what day of the year it is out of 365 with 1 being the first day of the year and 365 being the last (for example: 142). Your method should calculate and return what day of the week the given day falls on. You should use integers to represent the days of the week (0 being Sunday, 1 being Monday, 2 being Tuesday, 6 being Saturday). You should assume that the first day of the year is a Wednesday. Below are some example method calls and the value they would return:

dayorWeek(5): 0
dayorWeek(63): 2
dayorWeek(247): 4
dayorWeek(316): 3

The Correct Answer and Explanation is:

Here is the correct implementation of the dayorWeek method in Java:

public class DayOfWeekCalculator {
    public static int dayorWeek(int dayOfYear) {
        // First day of the year is a Wednesday, which is 3
        int startDay = 3;

        // Subtract 1 because day 1 is already the starting day
        int dayOfWeek = (startDay + (dayOfYear - 1)) % 7;

        return dayOfWeek;
    }

    // Optional main method for testing
    public static void main(String[] args) {
        System.out.println(dayorWeek(5));   // Output: 0 (Sunday)
        System.out.println(dayorWeek(63));  // Output: 2 (Tuesday)
        System.out.println(dayorWeek(247)); // Output: 4 (Thursday)
        System.out.println(dayorWeek(316)); // Output: 3 (Wednesday)
    }
}

✅ Explanation (300+ words):

The method dayorWeek is designed to determine the day of the week corresponding to a given day of the year (1 through 365). The days of the week are represented numerically, where:

  • 0 → Sunday
  • 1 → Monday
  • 2 → Tuesday
  • 3 → Wednesday
  • 4 → Thursday
  • 5 → Friday
  • 6 → Saturday

The critical point in this problem is that day 1 of the year is a Wednesday, which corresponds to 3 in the day-of-week numbering.

To calculate what day of the week any given day falls on, you use modulo arithmetic. The idea is to count how many days have passed since day 1 and adjust the weekday accordingly. This is done using the formula:

(startDay + (dayOfYear - 1)) % 7

Why subtract 1?

If dayOfYear is 1, we are on the first day of the year, so (dayOfYear - 1) is 0. We do not want to move forward at all. That way, the result is simply the startDay, which is 3 (Wednesday). This ensures the calculation is accurate without overshooting by one day.

Example:

dayorWeek(5)
→ Days passed since Jan 1: 5 - 1 = 4
→ (3 + 4) % 7 = 7 % 7 = 0
→ Day of the week: 0 (Sunday)

dayorWeek(63)
→ (3 + 62) % 7 = 65 % 7 = 2
→ Day of the week: 2 (Tuesday)

This method provides a simple and reliable way to compute weekdays from ordinal days using modular arithmetic, assuming the starting weekday is known.

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