diff --git a/src/day.rs b/src/day.rs
index f1486e5..0f7325c 100644
--- a/src/day.rs
+++ b/src/day.rs
@@ -27,8 +27,9 @@ impl Day {
         Some(Self(day))
     }
 
-    fn new_unchecked(day: u8) -> Self {
-        debug_assert!(day != 0 && day <= 25);
+    // Not part of the public API
+    #[doc(hidden)]
+    pub const fn __new_unchecked(day: u8) -> Self {
         Self(day)
     }
 
@@ -99,7 +100,7 @@ impl Iterator for EveryDay {
             return None;
         }
         // NOTE: the iterator start at 1 and we have verified that the value is not above 25.
-        let day = Day::new_unchecked(self.current);
+        let day = Day(self.current);
         self.current += 1;
 
         Some(day)
@@ -108,6 +109,24 @@ impl Iterator for EveryDay {
 
 /* -------------------------------------------------------------------------- */
 
+/// Crates a [`Day`] value in a const context.
+#[macro_export]
+macro_rules! day {
+    ($day:expr) => {{
+        const _ASSERT: () = assert!(
+            $day != 0 && $day <= 25,
+            concat!(
+                "invalid day number `",
+                $day,
+                "`, expecting a value between 1 and 25"
+            ),
+        );
+        $crate::Day::__new_unchecked($day)
+    }};
+}
+
+/* -------------------------------------------------------------------------- */
+
 #[cfg(feature = "test_lib")]
 mod tests {
     use super::{every_day, Day};
diff --git a/src/template/commands/scaffold.rs b/src/template/commands/scaffold.rs
index d4df4b9..382283d 100644
--- a/src/template/commands/scaffold.rs
+++ b/src/template/commands/scaffold.rs
@@ -14,7 +14,7 @@ pub fn part_two(input: &str) -> Option<u32> {
     None
 }
 
-advent_of_code::main!(DAY);
+advent_of_code::main!(DAY_NUMBER);
 
 #[cfg(test)]
 mod tests {
@@ -55,7 +55,11 @@ pub fn handle(day: Day) {
         }
     };
 
-    match file.write_all(MODULE_TEMPLATE.replace("DAY", &day.to_string()).as_bytes()) {
+    match file.write_all(
+        MODULE_TEMPLATE
+            .replace("DAY_NUMBER", &day.into_inner().to_string())
+            .as_bytes(),
+    ) {
         Ok(()) => {
             println!("Created module file \"{}\"", &module_path);
         }
diff --git a/src/template/mod.rs b/src/template/mod.rs
index faddf0a..215f039 100644
--- a/src/template/mod.rs
+++ b/src/template/mod.rs
@@ -1,3 +1,4 @@
+use crate::Day;
 use std::{env, fs};
 
 pub mod aoc_cli;
@@ -10,12 +11,10 @@ pub const ANSI_BOLD: &str = "\x1b[1m";
 pub const ANSI_RESET: &str = "\x1b[0m";
 
 /// Helper function that reads a text file to a string.
-#[must_use] pub fn read_file(folder: &str, day: u8) -> String {
+#[must_use]
+pub fn read_file(folder: &str, day: Day) -> String {
     let cwd = env::current_dir().unwrap();
-    let filepath = cwd
-        .join("data")
-        .join(folder)
-        .join(format!("{day:02}.txt"));
+    let filepath = cwd.join("data").join(folder).join(format!("{day}.txt"));
     let f = fs::read_to_string(filepath);
     f.expect("could not open input file")
 }
@@ -24,11 +23,14 @@ pub const ANSI_RESET: &str = "\x1b[0m";
 #[macro_export]
 macro_rules! main {
     ($day:expr) => {
+        /// The current day.
+        const DAY: advent_of_code::Day = advent_of_code::day!($day);
+
         fn main() {
             use advent_of_code::template::runner::*;
-            let input = advent_of_code::template::read_file("inputs", $day);
-            run_part(part_one, &input, $day, 1);
-            run_part(part_two, &input, $day, 2);
+            let input = advent_of_code::template::read_file("inputs", DAY);
+            run_part(part_one, &input, DAY, 1);
+            run_part(part_two, &input, DAY, 2);
         }
     };
 }