Step by Step solution about How to resolve the algorithm Day of the week step by step in the C programming language

Overview: The provided code snippet calculates the day of the week (Sunday = 0) for a given date using the proleptic Gregorian calendar. It then prints the dates when December 25th falls on a Sunday in the range of years 2008 to 2121.

Detailed Breakdown:

wday Function:

• Calculates the day of the week for a given date using the proleptic Gregorian calendar.
• It takes three arguments: year (y), month (m), and day (d).
• It first calculates an adjustment based on the month, then adjusts the month (mm) and year (yy) accordingly.
• It then uses Zeller's congruence formula to calculate the day of the week:
  • day + (13 * mm - 1) / 5 + yy + yy / 4 - yy / 100 + yy / 400
• This formula takes into account the day, month, year, leap year, and century leap year adjustments.
• The result is returned as the day of the week modulo 7 (Sunday = 0).

main Function:

• Defines an integer variable y used to iterate through years.
• Uses a for loop to iterate from year 2008 to 2121.
• For each year, it calls the wday function to calculate the day of the week for December 25th.
• If December 25th is a Sunday (day of week = 0), it prints the year in the format "YYYY-12-25".

Output: The code prints the following dates when December 25th falls on a Sunday in the range of years 2008 to 2121:

• 2008-12-25
• 2019-12-25

#include <stdio.h>

/* Calculate day of week in proleptic Gregorian calendar. Sunday == 0. */
int wday(int year, int month, int day)
{
	int adjustment, mm, yy;

	adjustment = (14 - month) / 12;
	mm = month + 12 * adjustment - 2;
	yy = year - adjustment;
	return (day + (13 * mm - 1) / 5 +
		yy + yy / 4 - yy / 100 + yy / 400) % 7;
}

int main()
{
	int y;

	for (y = 2008; y <= 2121; y++) {
		if (wday(y, 12, 25) == 0) printf("%04d-12-25\n", y);
	}

	return 0;
}