diff --git a/exchange_calendars/exchange_calendar.py b/exchange_calendars/exchange_calendar.py
index e57127ac..301b6c14 100644
--- a/exchange_calendars/exchange_calendar.py
+++ b/exchange_calendars/exchange_calendar.py
@@ -357,11 +357,15 @@ def __init__(
         _special_opens = self._calculate_special_opens(start, end)
         _special_closes = self._calculate_special_closes(start, end)
 
-        # Overwrite the special opens and closes on top of the standard ones.
-        _overwrite_special_dates(_all_days, self._opens, _special_opens)
-        _overwrite_special_dates(_all_days, self._closes, _special_closes)
-        _remove_breaks_for_special_dates(_all_days, self._break_starts, _special_closes)
-        _remove_breaks_for_special_dates(_all_days, self._break_ends, _special_closes)
+        # Adjust for special opens and closes.
+        self._opens = _adjust_special_dates(_all_days, self._opens, _special_opens)
+        self._closes = _adjust_special_dates(_all_days, self._closes, _special_closes)
+        self._break_starts = _remove_breaks_for_special_dates(
+            _all_days, self._break_starts, _special_closes
+        )
+        self._break_ends = _remove_breaks_for_special_dates(
+            _all_days, self._break_ends, _special_closes
+        )
 
         break_starts = None if self._break_starts is None else self._break_starts
         break_ends = None if self._break_ends is None else self._break_ends
@@ -2869,18 +2873,18 @@ def scheduled_special_times(
     )
 
 
-def _overwrite_special_dates(
+def _adjust_special_dates(
     session_labels: pd.DatetimeIndex,
     standard_times: pd.DatetimeIndex,
     special_times: pd.Series,
-) -> None:
-    """Overwrite standard times of a session bound with special times.
+) -> pd.DatetimeIndex:
+    """Adjust standard times of a session bound with special times.
 
     `session_labels` required for alignment.
     """
     # Short circuit when nothing to apply.
     if special_times.empty:
-        return
+        return standard_times
 
     len_m, len_oc = len(session_labels), len(standard_times)
     if len_m != len_oc:
@@ -2900,21 +2904,19 @@ def _overwrite_special_dates(
         bad_dates = list(special_times[indexer == -1])
         raise ValueError(f"Special dates {bad_dates} are not sessions.")
 
-    # NOTE: This is a slightly dirty hack.  We're in-place overwriting the
-    # internal data of an Index, which is conceptually immutable.  Since we're
-    # maintaining sorting, this should be ok, but this is a good place to
-    # sanity check if things start going haywire with calendar computations.
-    standard_times.values[indexer] = special_times.values
+    srs = standard_times.to_series()
+    srs.iloc[indexer] = special_times
+    return pd.DatetimeIndex(srs)
 
 
 def _remove_breaks_for_special_dates(
     session_labels: pd.DatetimeIndex,
     standard_break_times: pd.DatetimeIndex | None,
     special_times: pd.Series,
-) -> None:
+) -> pd.DatetimeIndex | None:
     """Remove standard break times for sessions with special times."
 
-    Overwrites standard break times with NaT for sessions with speical
+    Replaces standard break times with NaT for sessions with speical
     times. Anticipated that `special_times` will be special times for
     'opens' or 'closes'.
 
@@ -2922,11 +2924,11 @@ def _remove_breaks_for_special_dates(
     """
     # Short circuit when we have no breaks
     if standard_break_times is None:
-        return
+        return None
 
     # Short circuit when nothing to apply.
     if special_times.empty:
-        return
+        return standard_break_times
 
     len_m, len_oc = len(session_labels), len(standard_break_times)
     if len_m != len_oc:
@@ -2946,8 +2948,6 @@ def _remove_breaks_for_special_dates(
         bad_dates = list(special_times[indexer == -1])
         raise ValueError(f"Special dates {bad_dates} are not trading days.")
 
-    # NOTE: This is a slightly dirty hack.  We're in-place overwriting the
-    # internal data of an Index, which is conceptually immutable.  Since we're
-    # maintaining sorting, this should be ok, but this is a good place to
-    # sanity check if things start going haywire with calendar computations.
-    standard_break_times.values[indexer] = NP_NAT
+    srs = standard_break_times.to_series()
+    srs.iloc[indexer] = np.nan
+    return pd.DatetimeIndex(srs)
diff --git a/exchange_calendars/exchange_calendar_xhkg.py b/exchange_calendars/exchange_calendar_xhkg.py
index 35e54637..c29f188a 100644
--- a/exchange_calendars/exchange_calendar_xhkg.py
+++ b/exchange_calendars/exchange_calendar_xhkg.py
@@ -232,7 +232,9 @@ def process_queen_birthday(dt):
     #  on the Monday (2022-09-12). In the past they don't seem to have followed
     #  this pattern. We'll have to wait and see before we generalise this into a rule.
     pd.Timestamp("2022-09-12"),
-    pd.Timestamp("2023-07-17"),  # 8号台风泰利, 全天休市 https://www.hkex.com.hk/News/Market-Communications/2023/2307172news?sc_lang=en
+    pd.Timestamp(
+        "2023-07-17"
+    ),  # 8号台风泰利, 全天休市 https://www.hkex.com.hk/News/Market-Communications/2023/2307172news?sc_lang=en
 ]
 
 
@@ -359,7 +361,7 @@ def adhoc_holidays(self):
 
         qingming_festival = vectorized_sunday_to_monday(
             qingming_festival_dates,
-        ).values
+        ).values.copy()  # copy so that array is writeable
         years = qingming_festival.astype("M8[Y]")
         easter_monday = EasterMonday.dates(years[0], years[-1] + 1)
         # qingming gets observed one day later if easter monday is on the same
@@ -370,7 +372,7 @@ def adhoc_holidays(self):
         # conflicts with national day, then national day is observed on the
         # second, though we don't encode that in the regular holiday, so
         # instead we pretend that the mid autumn festival would be delayed.
-        mid_autumn_festival = day_after_mid_autumn_festival_dates.values
+        mid_autumn_festival = day_after_mid_autumn_festival_dates.values.copy()
         mid_autumn_festival[
             (day_after_mid_autumn_festival_dates.month == 10)
             & (day_after_mid_autumn_festival_dates.day == 1)
@@ -435,7 +437,7 @@ def special_closes(self):
     @property
     def special_closes_adhoc(self):
         lunar_new_years_eve = (chinese_lunar_new_year_dates - pd.Timedelta(days=1))[
-            np.in1d(
+            np.isin(
                 chinese_lunar_new_year_dates.weekday,
                 [TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY],
             )
diff --git a/exchange_calendars/exchange_calendar_xkrx.py b/exchange_calendars/exchange_calendar_xkrx.py
index 846e79b2..1bd6a1cb 100644
--- a/exchange_calendars/exchange_calendar_xkrx.py
+++ b/exchange_calendars/exchange_calendar_xkrx.py
@@ -217,7 +217,7 @@ def special_offsets_adhoc(
             ),
         ]
 
-    def _overwrite_special_offsets(
+    def _adjust_special_offsets(
         self,
         session_labels: pd.DatetimeIndex,
         standard_times: pd.DatetimeIndex | None,
@@ -229,7 +229,7 @@ def _overwrite_special_offsets(
     ):
         # Short circuit when nothing to apply.
         if standard_times is None or not len(standard_times):
-            return
+            return standard_times
 
         len_m, len_oc = len(session_labels), len(standard_times)
         if len_m != len_oc:
@@ -271,13 +271,11 @@ def _overwrite_special_offsets(
 
         # Short circuit when nothing to apply.
         if not len(special_opens_or_closes):
-            return
+            return standard_times
 
-        # NOTE: This is a slightly dirty hack.  We're in-place overwriting the
-        # internal data of an Index, which is conceptually immutable.  Since we're
-        # maintaining sorting, this should be ok, but this is a good place to
-        # sanity check if things start going haywire with calendar computations.
-        standard_times.values[indexer] = special_opens_or_closes.values
+        srs = standard_times.to_series()
+        srs.iloc[indexer] = special_opens_or_closes
+        return pd.DatetimeIndex(srs)
 
     def apply_special_offsets(
         self,
@@ -314,7 +312,7 @@ def apply_special_offsets(
             (t[3], t[-1]) for t in _special_offsets_adhoc if t[3] is not None
         ]
 
-        self._overwrite_special_offsets(
+        self._opens = self._adjust_special_offsets(
             session_labels,
             self._opens,
             _special_open_offsets,
@@ -322,7 +320,7 @@ def apply_special_offsets(
             start,
             end,
         )
-        self._overwrite_special_offsets(
+        self._break_starts = self._adjust_special_offsets(
             session_labels,
             self._break_starts,
             _special_break_start_offsets,
@@ -330,7 +328,7 @@ def apply_special_offsets(
             start,
             end,
         )
-        self._overwrite_special_offsets(
+        self._break_ends = self._adjust_special_offsets(
             session_labels,
             self._break_ends,
             _special_break_end_offsets,
@@ -338,7 +336,7 @@ def apply_special_offsets(
             start,
             end,
         )
-        self._overwrite_special_offsets(
+        self._closes = self._adjust_special_offsets(
             session_labels,
             self._closes,
             _special_close_offsets,
diff --git a/tests/test_exchange_calendar.py b/tests/test_exchange_calendar.py
index c2f12f37..3ce62c71 100644
--- a/tests/test_exchange_calendar.py
+++ b/tests/test_exchange_calendar.py
@@ -1364,7 +1364,7 @@ def _trading_minute_to_break_minute(
         for session, break_session in zip(sessions[mask], break_sessions[mask]):
             break_minutes = self.get_session_break_minutes(break_session)
             trading_minutes = self.get_session_minutes(session)[0]
-            bv = np.in1d(trading_minutes.time, break_minutes.time)
+            bv = np.isin(trading_minutes.time, break_minutes.time)
             minutes.append([trading_minutes[bv][-1], session, break_session])
         return minutes