Day of the week

Math

59.0% acceptance

Feb 25, 2026

452

2559

Given a date, return the corresponding day of the week for that date. The input is given as three integers representing the day, month and year respectively. Return the answer as one of the following values {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"}. Note: January 1, 1971 was a Friday.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn day_of_the_week(day: i32, month: i32, year: i32) -> String {
    // Zeller's congruence or day count from known reference
    // January 1, 1971 was Friday (index 4: Sun=0,Mon=1,...Fri=5,Sat=6)
    let days = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"];
    // Count total days from 1971-01-01
    let is_leap = |y: i32| (y % 4 == 0 && y % 100 != 0) || y % 400 == 0;
    let month_days = [0i32, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
    let mut total = 0i32;
    for y in 1971..year {
      total += if is_leap(y) { 366 } else { 365 };
    }
    for m in 1..month {
      total += month_days[m as usize];
      if m == 2 && is_leap(year) { total += 1; }
    }
    total += day - 1;
    // 1971-01-01 is Friday = index 5
    let idx = (5 + total).rem_euclid(7) as usize;
    days[idx].to_string()
  }
}