diff --git a/composer.json b/composer.json
index 57ce310..153aa9d 100644
--- a/composer.json
+++ b/composer.json
@@ -40,6 +40,11 @@
         }
     },
     "scripts": {
+        "build": [
+            "@static:analyze",
+            "@test",
+            "@test:mutation"
+        ],
         "test": [
             "phpunit"
         ],
diff --git a/composer.lock b/composer.lock
index f6058f1..241ce35 100644
--- a/composer.lock
+++ b/composer.lock
@@ -12,7 +12,7 @@
             "source": {
                 "type": "git",
                 "url": "git@github.com:aeon-php/calendar.git",
-                "reference": "32116e9f3fd7a893cb9b5cdb499660f2bcf0ac66"
+                "reference": "b19db0d696f866e392b977abab0649105e9bd08f"
             },
             "require": {
                 "php": ">=7.4.2",
@@ -72,7 +72,7 @@
                 "immutable",
                 "sleep"
             ],
-            "time": "2020-05-31T21:46:37+00:00"
+            "time": "2020-06-02T20:41:06+00:00"
         },
         {
             "name": "symfony/polyfill-ctype",
@@ -2582,21 +2582,21 @@
         },
         {
             "name": "phpstan/phpstan-webmozart-assert",
-            "version": "0.12.4",
+            "version": "0.12.5",
             "source": {
                 "type": "git",
                 "url": "https://github.com/phpstan/phpstan-webmozart-assert.git",
-                "reference": "714e7a6b936931790855c8da958153dfb25fc953"
+                "reference": "d1644c76cefeec4cf12c609c75f60b0f11b511d7"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/phpstan/phpstan-webmozart-assert/zipball/714e7a6b936931790855c8da958153dfb25fc953",
-                "reference": "714e7a6b936931790855c8da958153dfb25fc953",
+                "url": "https://api.github.com/repos/phpstan/phpstan-webmozart-assert/zipball/d1644c76cefeec4cf12c609c75f60b0f11b511d7",
+                "reference": "d1644c76cefeec4cf12c609c75f60b0f11b511d7",
                 "shasum": ""
             },
             "require": {
                 "php": "~7.1",
-                "phpstan/phpstan": "^0.12.6"
+                "phpstan/phpstan": "^0.12.24"
             },
             "require-dev": {
                 "consistence/coding-standard": "^3.7",
@@ -2631,7 +2631,7 @@
                 "MIT"
             ],
             "description": "PHPStan webmozart/assert extension",
-            "time": "2020-05-04T08:30:18+00:00"
+            "time": "2020-06-02T20:02:11+00:00"
         },
         {
             "name": "phpunit/php-code-coverage",
diff --git a/src/Aeon/Calendar/Gregorian/Holidays.php b/src/Aeon/Calendar/Gregorian/Holidays.php
index 9745ff5..982d973 100644
--- a/src/Aeon/Calendar/Gregorian/Holidays.php
+++ b/src/Aeon/Calendar/Gregorian/Holidays.php
@@ -6,6 +6,9 @@
 
 use Aeon\Calendar\Gregorian\Holidays\Holiday;
 
+/**
+ * @psalm-immutable
+ */
 interface Holidays
 {
     public function isHoliday(Day $day) : bool;
diff --git a/src/Aeon/Calendar/Gregorian/Holidays/GoogleCalendarRegionalHolidays.php b/src/Aeon/Calendar/Gregorian/Holidays/GoogleCalendarRegionalHolidays.php
index f89fb53..71dc38e 100644
--- a/src/Aeon/Calendar/Gregorian/Holidays/GoogleCalendarRegionalHolidays.php
+++ b/src/Aeon/Calendar/Gregorian/Holidays/GoogleCalendarRegionalHolidays.php
@@ -9,6 +9,9 @@
 use Aeon\Calendar\Gregorian\Holidays;
 use Webmozart\Assert\Assert;
 
+/**
+ * @psalm-immutable
+ */
 final class GoogleCalendarRegionalHolidays implements Holidays
 {
     /**
@@ -38,6 +41,11 @@ function (string $countryCode) : string {
         $this->calendars = null;
     }
 
+    /**
+     * @param Day $day
+     * @return bool
+     * @throws HolidayYearException
+     */
     public function isHoliday(Day $day) : bool
     {
         if ($this->calendars === null) {
@@ -100,6 +108,7 @@ private function loadCalendars() : void
         }
     }
 
+    /** @psalm-suppress InaccessibleProperty */
     private function loadCalendar(string $countryCode) : void
     {
         /**