From 6f9d5da45d58e6f8253583130506e69b36e6faf8 Mon Sep 17 00:00:00 2001
From: Anish Mehta <mehtaanish111@gmail.com>
Date: Fri, 30 Oct 2026 22:01:08 +0530
Subject: [PATCH] fix(feature_flags): match days of week regardless of locale

---
 .../utilities/feature_flags/comparators.py    | 16 ++++++-
 .../_boto3/test_time_based_actions.py         | 42 +++++++++++++++++++
 2 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/aws_lambda_powertools/utilities/feature_flags/comparators.py b/aws_lambda_powertools/utilities/feature_flags/comparators.py
index 2888c1cb14b..a7f4617f71e 100644
--- a/aws_lambda_powertools/utilities/feature_flags/comparators.py
+++ b/aws_lambda_powertools/utilities/feature_flags/comparators.py
@@ -19,11 +19,23 @@ def _get_now_from_timezone(timezone: tzinfo | None) -> datetime:
     return datetime.now(timezone)
 
 
+# Indexed by datetime.weekday(), where Monday is 0
+_DAYS_OF_WEEK = (
+    TimeValues.MONDAY.value,
+    TimeValues.TUESDAY.value,
+    TimeValues.WEDNESDAY.value,
+    TimeValues.THURSDAY.value,
+    TimeValues.FRIDAY.value,
+    TimeValues.SATURDAY.value,
+    TimeValues.SUNDAY.value,
+)
+
+
 def compare_days_of_week(context_value: Any, condition_value: dict) -> bool:
     timezone_name = condition_value.get(TimeValues.TIMEZONE.value, "UTC")
 
-    # %A = Weekday as locale’s full name.
-    current_day = _get_now_from_timezone(gettz(timezone_name)).strftime("%A").upper()
+    # Not strftime("%A"): it names the day in the process locale, e.g. "FREITAG" under a German LC_TIME
+    current_day = _DAYS_OF_WEEK[_get_now_from_timezone(gettz(timezone_name)).weekday()]
 
     days = condition_value.get(TimeValues.DAYS.value, [])
     return current_day in days
diff --git a/tests/functional/feature_flags/_boto3/test_time_based_actions.py b/tests/functional/feature_flags/_boto3/test_time_based_actions.py
index 36ea41cf8f6..48fcb9767fe 100644
--- a/tests/functional/feature_flags/_boto3/test_time_based_actions.py
+++ b/tests/functional/feature_flags/_boto3/test_time_based_actions.py
@@ -416,6 +416,48 @@ def test_time_based_utc_days_range_rule_match(mocker):
     )
 
 
+def test_time_based_days_range_rule_match_with_non_english_locale(mocker):
+    # GIVEN a process whose LC_TIME names the days in German, so strftime("%A") on a Friday is "Freitag"
+    class GermanDatetime(dt.datetime):
+        def strftime(self, fmt):
+            return super().strftime(fmt).replace("Friday", "Freitag")
+
+    friday = GermanDatetime(2022, 11, 18, 10, 0, 0, tzinfo=dt.timezone.utc)
+    mocker.patch(
+        "aws_lambda_powertools.utilities.feature_flags.comparators._get_now_from_timezone",
+        return_value=friday,
+    )
+    mocked_get_conf = mocker.patch("aws_lambda_powertools.utilities.parameters.AppConfigProvider.get")
+    mocked_get_conf.return_value = {
+        "my_feature": {
+            FEATURE_DEFAULT_VAL_KEY: False,
+            RULES_KEY: {
+                "match only on fridays": {
+                    RULE_MATCH_VALUE: True,
+                    CONDITIONS_KEY: [
+                        {
+                            CONDITION_ACTION: RuleAction.SCHEDULE_BETWEEN_DAYS_OF_WEEK.value,
+                            CONDITION_KEY: TimeKeys.CURRENT_DAY_OF_WEEK.value,
+                            CONDITION_VALUE: {TimeValues.DAYS.value: [TimeValues.FRIDAY.value]},
+                        },
+                    ],
+                },
+            },
+        },
+    }
+    store = AppConfigStore(
+        environment="test_env",
+        application="test_app",
+        name="test_conf_name",
+        max_age=600,
+        sdk_config=Config(region_name="us-east-1"),
+    )
+
+    # WHEN evaluating the rule
+    # THEN the day still matches FRIDAY
+    assert FeatureFlags(store=store).evaluate(name="my_feature", context={}, default=False)
+
+
 def test_time_based_utc_days_range_no_rule_match(mocker):
     assert not evaluate_mocked_schema(
         mocker=mocker,