diff --git a/OWNd/message/heating.py b/OWNd/message/heating.py
index a9fda25..6c92328 100644
--- a/OWNd/message/heating.py
+++ b/OWNd/message/heating.py
@@ -186,6 +186,7 @@ def __init__(self, data: str) -> None:
         self._holiday_end_date: tuple[int, int, int] | None = None
         self._holiday_end_time: tuple[int, int] | None = None
         self._manual_timed_duration: tuple[int, int] | None = None
+        self._holiday_days: int | None = None
 
         self._is_active = None
         self._is_heating = None
@@ -211,9 +212,14 @@ def __init__(self, data: str) -> None:
                 self._mode in [0, 210, 211, 212, 215]
                 or (self._mode >= 2101 and self._mode <= 2103)
                 or (self._mode >= 2201 and self._mode <= 2216)
+                or (self._mode >= 23001 and self._mode <= 23255)
             ):
+                # 23xxx: holiday days in conditioning mode (Legrand WHO 4
+                # p. 5 / p. 64; libqtdevices thermal_device.cpp:58, 258-262).
                 self._type = MESSAGE_TYPE_MODE
                 self._mode_name = CLIMATE_MODE_COOL
+                if self._mode >= 23001:
+                    self._holiday_days = self._mode % 1000
                 self._human_readable_log = (
                     f"Zone {self._zone}'s mode is set to '{self._mode_name}'"
                 )
@@ -221,9 +227,14 @@ def __init__(self, data: str) -> None:
                 self._mode in [1, 110, 111, 112, 115]
                 or (self._mode >= 1101 and self._mode <= 1103)
                 or (self._mode >= 1201 and self._mode <= 1216)
+                or (self._mode >= 13001 and self._mode <= 13255)
             ):
+                # 13xxx: holiday days in heating mode (Legrand WHO 4 p. 5 /
+                # p. 64; libqtdevices thermal_device.cpp:68, 303-307).
                 self._type = MESSAGE_TYPE_MODE
                 self._mode_name = CLIMATE_MODE_HEAT
+                if self._mode >= 13001:
+                    self._holiday_days = self._mode % 1000
                 self._human_readable_log = (
                     f"Zone {self._zone}'s mode is set to '{self._mode_name}'"
                 )
@@ -232,11 +243,11 @@ def __init__(self, data: str) -> None:
                 or (self._mode >= 3101 and self._mode <= 3116)
                 or (self._mode >= 3201 and self._mode <= 3216)
                 or (self._mode >= 33001 and self._mode <= 33255)
-                or (self._mode >= 23001 and self._mode <= 23255)
-                or (self._mode >= 13001 and self._mode <= 13255)
             ):
                 self._type = MESSAGE_TYPE_MODE
                 self._mode_name = CLIMATE_MODE_AUTO
+                if self._mode >= 33001:
+                    self._holiday_days = self._mode % 1000
                 self._human_readable_log = (
                     f"Zone {self._zone}'s mode is set to '{self._mode_name}'"
                 )
@@ -662,6 +673,11 @@ def holiday_end_date(self) -> tuple[int, int, int] | None:
         """Holiday / weekend end date (day, month, year), or None."""
         return self._holiday_end_date
 
+    @property
+    def holiday_days(self) -> int | None:
+        """Holiday days (1..255) named by a 13xxx/23xxx/33xxx WHAT, else None."""
+        return self._holiday_days
+
     @property
     def holiday_end_time(self) -> tuple[int, int] | None:
         """Holiday / weekend end time (hour, minute), or None."""
diff --git a/tests/test_message_edges.py b/tests/test_message_edges.py
index 4d3ac4d..03179d1 100644
--- a/tests/test_message_edges.py
+++ b/tests/test_message_edges.py
@@ -196,15 +196,32 @@ def test_mode_auto_variant_315(self):
         assert isinstance(msg, OWNHeatingEvent)
         assert msg.mode == CLIMATE_MODE_AUTO
 
-    def test_mode_auto_weekly_23001(self):
+    def test_mode_holiday_23001_is_conditioning(self):
+        # Legrand WHO 4 p. 5 and p. 64: 23xxx = holiday days in conditioning
+        # mode; libqtdevices thermal_device.cpp:58 (SUM_HOLIDAY = 23000) and
+        # :258-262 (season SE_SUMMER).
         msg = OWNEvent.parse("*4*23001*1##")
         assert isinstance(msg, OWNHeatingEvent)
-        assert msg.mode == CLIMATE_MODE_AUTO
+        assert msg.mode == CLIMATE_MODE_COOL
+        assert msg.holiday_days == 1
 
-    def test_mode_auto_weekly_13001(self):
+    def test_mode_holiday_13001_is_heating(self):
+        # Legrand WHO 4 p. 5 and p. 64: 13xxx = holiday days in heating mode;
+        # libqtdevices thermal_device.cpp:68 (WIN_HOLIDAY = 13000) and
+        # :303-307 (season SE_WINTER).
         msg = OWNEvent.parse("*4*13001*1##")
         assert isinstance(msg, OWNHeatingEvent)
+        assert msg.mode == CLIMATE_MODE_HEAT
+        assert msg.holiday_days == 1
+
+    def test_mode_holiday_33004_is_generic(self):
+        msg = OWNEvent.parse("*4*33004*#0##")
         assert msg.mode == CLIMATE_MODE_AUTO
+        assert msg.holiday_days == 4
+
+    def test_holiday_days_is_none_for_other_modes(self):
+        assert OWNEvent.parse("*4*1*1##").holiday_days is None
+        assert OWNEvent.parse("*4*3101*#0##").holiday_days is None
 
     def test_unknown_mode(self):
         msg = OWNEvent.parse("*4*999*1##")