diff --git a/modules/module-list.nix b/modules/module-list.nix
index f0b1d3d..15f6f39 100644
--- a/modules/module-list.nix
+++ b/modules/module-list.nix
@@ -23,6 +23,7 @@
   ./system/defaults/dock.nix
   ./system/defaults/finder.nix
   ./system/defaults/hitoolbox.nix
+  ./system/defaults/iCal.nix
   ./system/defaults/screencapture.nix
   ./system/defaults/screensaver.nix
   ./system/defaults/alf.nix
diff --git a/modules/system/defaults-write.nix b/modules/system/defaults-write.nix
index a00b0e4..41911a9 100644
--- a/modules/system/defaults-write.nix
+++ b/modules/system/defaults-write.nix
@@ -27,6 +27,7 @@ let
   dock = defaultsToList "com.apple.dock" dockFiltered;
   finder = defaultsToList "com.apple.finder" cfg.finder;
   hitoolbox = defaultsToList "com.apple.HIToolbox" cfg.hitoolbox;
+  iCal = defaultsToList "com.apple.iCal" cfg.iCal;
   magicmouse = defaultsToList "com.apple.AppleMultitouchMouse" cfg.magicmouse;
   magicmouseBluetooth = defaultsToList "com.apple.driver.AppleMultitouchMouse.mouse" cfg.magicmouse;
   screencapture = defaultsToList "com.apple.screencapture" cfg.screencapture;
@@ -37,7 +38,7 @@ let
   universalaccess = defaultsToList "com.apple.universalaccess" cfg.universalaccess;
   ActivityMonitor = defaultsToList "com.apple.ActivityMonitor" cfg.ActivityMonitor;
   WindowManager = defaultsToList "com.apple.WindowManager" cfg.WindowManager;
-  controlcenter = defaultsToList "~/Library/Preferences/ByHost/com.apple.controlcenter" cfg.controlcenter;  
+  controlcenter = defaultsToList "~/Library/Preferences/ByHost/com.apple.controlcenter" cfg.controlcenter;
   CustomUserPreferences = flatten (mapAttrsToList (name: value: defaultsToList name value) cfg.CustomUserPreferences);
   CustomSystemPreferences = flatten (mapAttrsToList (name: value: defaultsToList name value) cfg.CustomSystemPreferences);
 
@@ -83,6 +84,7 @@ in
         dock
         finder
         hitoolbox
+        iCal
         magicmouse
         magicmouseBluetooth
         screencapture
@@ -108,6 +110,7 @@ in
         ${concatStringsSep "\n" dock}
         ${concatStringsSep "\n" finder}
         ${concatStringsSep "\n" hitoolbox}
+        ${concatStringsSep "\n" iCal}
         ${concatStringsSep "\n" magicmouse}
         ${concatStringsSep "\n" magicmouseBluetooth}
         ${concatStringsSep "\n" screencapture}
diff --git a/modules/system/defaults/iCal.nix b/modules/system/defaults/iCal.nix
new file mode 100644
index 0000000..13f1458
--- /dev/null
+++ b/modules/system/defaults/iCal.nix
@@ -0,0 +1,52 @@
+{ lib, ... }:
+
+{
+  options = {
+    system.defaults.iCal."first day of week" = lib.mkOption {
+      type = lib.types.nullOr (lib.types.enum [
+        "System Setting"
+        "Sunday"
+        "Monday"
+        "Tuesday"
+        "Wednesday"
+        "Thursday"
+        "Friday"
+        "Saturday"
+      ]);
+      apply = key: if key == null then null else {
+        "System Setting" = 0;
+        "Sunday" = 1;
+        "Monday" = 2;
+        "Tuesday" = 3;
+        "Wednesday" = 4;
+        "Thursday" = 5;
+        "Friday" = 6;
+        "Saturday" = 7;
+      }.${key};
+      default = null;
+      description = ''
+        Set the day to start week on in the Calendar. The default is "System Setting".
+
+        System Setting means inherit the value from Language & Region.
+      '';
+    };
+
+    system.defaults.iCal.CalendarSidebarShown = lib.mkOption {
+      type = lib.types.nullOr lib.types.bool;
+      default = null;
+      description = ''
+        Show calendar list. The default is false.
+
+        This requires restarting `Calendar.app` to show.
+      '';
+    };
+
+    system.defaults.iCal."TimeZone support enabled" = lib.mkOption {
+      type = lib.types.nullOr lib.types.bool;
+      default = null;
+      description = ''
+        Turn on time zone support. The default is false.
+      '';
+    };
+  };
+}
diff --git a/tests/fixtures/system-defaults-write/activate-user.txt b/tests/fixtures/system-defaults-write/activate-user.txt
index d93321e..9f48668 100644
--- a/tests/fixtures/system-defaults-write/activate-user.txt
+++ b/tests/fixtures/system-defaults-write/activate-user.txt
@@ -458,6 +458,21 @@ defaults write com.apple.HIToolbox 'AppleFnUsageType' $'<?xml version="1.0" enco
 <plist version="1.0">
 <integer>2</integer>
 </plist>'
+defaults write com.apple.iCal 'CalendarSidebarShown' $'<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<true/>
+</plist>'
+defaults write com.apple.iCal 'TimeZone support enabled' $'<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<true/>
+</plist>'
+defaults write com.apple.iCal 'first day of week' $'<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<integer>4</integer>
+</plist>'
 
 
 defaults write com.apple.screencapture 'include-date' $'<?xml version="1.0" encoding="UTF-8"?>
diff --git a/tests/system-defaults-write.nix b/tests/system-defaults-write.nix
index 35ff853..6b06dd2 100644
--- a/tests/system-defaults-write.nix
+++ b/tests/system-defaults-write.nix
@@ -99,6 +99,9 @@
   system.defaults.finder.ShowMountedServersOnDesktop = false;
   system.defaults.finder.ShowRemovableMediaOnDesktop = false;
   system.defaults.hitoolbox.AppleFnUsageType = "Show Emoji & Symbols";
+  system.defaults.iCal."first day of week" = "Wednesday";
+  system.defaults.iCal.CalendarSidebarShown = true;
+  system.defaults.iCal."TimeZone support enabled" = true;
   system.defaults.screencapture.location = "/tmp";
   system.defaults.screencapture.target = "file";
   system.defaults.screencapture.include-date = true;