diff --git a/src/lib.rs b/src/lib.rs
index e142d69..7cb2af2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,9 +1,102 @@
-use crate::session_date_calculator::Day;
-use chrono::Weekday;
+use crate::session_date_calculator::{Day, NthWeekday};
+use chrono::{NaiveDate, Weekday};
+use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
 
 pub mod session_date_calculator;
 pub mod webpage;
 
+#[derive(Debug, Clone)]
+pub struct DatedSession {
+  pub day: Day,
+  pub session: Session,
+  pub applying_exception: Option<Exception>,
+}
+
+impl Noted for DatedSession {
+  fn note(&self) -> Option<&String> {
+    match &self.applying_exception {
+      Some(e) => e.note(),
+      None => self.session.note(),
+    }
+  }
+}
+
+#[derive(Debug, Clone, Eq, PartialEq, Serialize, Deserialize)]
+pub enum Session {
+  Regular {
+    rule: NthWeekday,
+    note: Option<String>,
+    except: HashMap<NaiveDate, Exception>,
+  },
+  Extra {
+    date: NaiveDate,
+    note: Option<String>,
+  },
+}
+
+impl Session {
+  pub fn into_dated(self, day: Day) -> Result<DatedSession, Self> {
+    if !self.is_applicable_to(&day) {
+      return Err(self);
+    }
+
+    let dated_session = DatedSession {
+      applying_exception: match self {
+        Session::Regular { ref except, .. } => except.get(&day.date).cloned(),
+        _ => None,
+      },
+      session: self,
+      day,
+    };
+    Ok(dated_session)
+  }
+
+  pub fn is_applicable_to(&self, day: &Day) -> bool {
+    match *self {
+      Session::Regular { rule, .. } => rule.matches(day),
+      Session::Extra { date, .. } => day.date == date,
+    }
+  }
+}
+
+impl Noted for Session {
+  fn note(&self) -> Option<&String> {
+    match self {
+      Session::Regular { note, .. } => note,
+      Session::Extra { note, .. } => note,
+    }
+    .as_ref()
+  }
+}
+
+#[derive(Debug, Clone, Eq, PartialEq, Hash, Serialize, Deserialize)]
+pub enum Exception {
+  Cancel {
+    note: Option<String>,
+  },
+  Alter {
+    #[serde(default)]
+    date: Option<NaiveDate>,
+    #[serde(default)]
+    note: Option<String>,
+  },
+}
+
+impl Noted for Exception {
+  fn note(&self) -> Option<&String> {
+    match self {
+      Exception::Cancel { note, .. } => note,
+      Exception::Alter { note, .. } => note,
+    }
+    .as_ref()
+  }
+}
+
+pub trait Noted {
+  fn note(&self) -> Option<&String>;
+}
+
 pub fn localize_day(day: &Day) -> String {
   format!(
     "{}, {}",
@@ -18,4 +111,4 @@ pub fn localize_day(day: &Day) -> String {
     },
     day.date.format("%d.%m.%Y")
   )
-}
\ No newline at end of file
+}
diff --git a/src/session_date_calculator.rs b/src/session_date_calculator.rs
index 01f1b7f..3d1e396 100644
--- a/src/session_date_calculator.rs
+++ b/src/session_date_calculator.rs
@@ -5,7 +5,7 @@ use std::fmt::{Display, Formatter};
 use std::num::ParseIntError;
 use std::str::FromStr;
 
-#[derive(Debug, Copy, Clone, Serialize, Deserialize)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Hash, Serialize, Deserialize)]
 pub struct NthWeekday {
   pub n: u8,
   pub weekday: Weekday,
diff --git a/styles.css b/styles.css
index ff5e3d3..35db1f9 100644
--- a/styles.css
+++ b/styles.css
@@ -5,7 +5,7 @@
   --white: white;
   --black: black;
   --red: red;
-  --yellow: #f0f00f;
+  --yellow: #bbbb11;
   --green: #1fd51f;
 }
 ul {
@@ -70,6 +70,9 @@ html, body {
 .highlight-background {
   background-color: var(--green);
 }
-.red-text {
-  color: var(--red);
-}
\ No newline at end of file
+.cancel-background {
+  background-color: var(--red);
+}
+.change-background {
+  background-color: var(--yellow);
+}