The below program in C will find out weekday based on the weekday number input. We will be following the below logic.
1 -> Sunday
2 -> Monday
3 -> Tuesday
4 -> Wednesday
5 -> Thursday
6 -> Friday
7 -> Saturday

Here we are accepting the week day number as input from the user and use switch statement to display the weekday.

Below is the program in C.

#include<stdio.h>
#include<conio.h>
void main(){
	int day;
	clrscr();
	printf("Enter the day number: ");
	scanf("%d", &day);
	switch(day){
		case 1:
			printf("\nIts Sunday");
			break;
		case 2:
			printf("\nIts Monday");
			break;
		case 3:
			printf("\nIts Tuesday");
			break;
		case 4:
			printf("\nIts Wednesday");
			break;
		case 5:
			printf("\nIts Thursday");
			break;
		case 6:
			printf("\nIts Friday");
			break;
		case 7:
			printf("\nIts Saturday");
			break;
		default:
			printf("\nPlease enter number between 1 to 7");
			break;
	}
	getch();
}

Output

Enter the day number: 5

Its Thursday