package main
import (
"fmt"
)
// Zeller's Congruence implementation
// Returns the day of the week for a given date.
func dayOfWeek(d int, m int, y int) string {
// Zeller's output mapping:
// 0 = Saturday, 1 = Sunday, 2 = Monday, ... 6 = Friday
names := []string{
"Saturday", "Sunday", "Monday", "Tuesday",
"Wednesday", "Thursday", "Friday",
}
// In Zeller's formula, January and February are counted
// as months 13 and 14 of the previous year.
if m < 3 {
m += 12 // Convert Jan → 13, Feb → 14
y -= 1 // Move to previous year
}
K := y % 100 // Year of the century (last two digits)
J := y / 100 // Zero-based century (e.g., 2024 → 20)
// Zeller's formula (clearer step-by-step version):
term1 := d // day of month
term2 := (13 * (m + 1)) / 5 // month adjustment
term3 := K // year of century
term4 := K / 4 // leap years in century
term5 := J / 4 // leap centuries
term6 := 5 * J // century correction
// Combine all terms and take modulo 7
h := (term1 + term2 + term3 + term4 + term5 + term6) % 7
// Return the corresponding weekday name
return names[h]
}
func main() {
d, m, y := 30, 5, 2024
fmt.Println(dayOfWeek(d, m, y))
}
/*
run:
Thursday
*/
