DEPR: correct error message in to_offset

pandas/_libs/tslibs/offsets.pyx

@@ -4735,6 +4735,9 @@ cpdef to_offset(freq, bint is_period=False):
                             f"for Period, please use \'Y{name[2:]}\' "
                             f"instead of \'{name}\'"
                         )
+                    if (name.startswith("B") or
+                            name.startswith("S") or name.startswith("C")):
+                        raise ValueError(INVALID_FREQ_ERR_MSG.format(name))
                     else:
                         raise ValueError(
                             f"for Period, please use "

pandas/tests/indexes/period/test_period.py

@@ -285,10 +285,11 @@ def test_map(self):
         "freq,freq_depr",
         [
             ("2M", "2ME"),
-            ("2SM", "2SME"),
+            ("2Q-MAR", "2QE-MAR"),
+            ("2Y-FEB", "2YE-FEB"),
         ],
     )
-    def test_period_index_frequency_ME_SME_error_message(self, freq, freq_depr):
+    def test_period_index_frequency_ME_error_message(self, freq, freq_depr):
         # GH#52064
         msg = f"for Period, please use '{freq[1:]}' instead of '{freq_depr[1:]}'"
 
@@ -316,11 +317,10 @@ def test_a_deprecated_from_time_series(self, freq_depr):
         series = Series(1, index=index)
         assert isinstance(series, Series)
 
-    @pytest.mark.parametrize("freq_depr", ["2ME", "2QE", "2BQE", "2YE"])
-    def test_period_index_frequency_error_message(self, freq_depr):
+    @pytest.mark.parametrize("freq_depr", ["2SME", "2CBME", "2BYE"])
+    def test_period_index_frequency_invalid_freq(self, freq_depr):
         # GH#9586
-        msg = f"for Period, please use '{freq_depr[1:-1]}' "
-        f"instead of '{freq_depr[1:]}'"
+        msg = f"Invalid frequency: {freq_depr[1:]}"
 
         with pytest.raises(ValueError, match=msg):
             period_range("2020-01", "2020-05", freq=freq_depr)

pandas/tests/resample/test_period_index.py

@@ -956,11 +956,8 @@ def test_resample_t_l_deprecated(self):
         ("2M", "2ME"),
         ("2Q", "2QE"),
         ("2Q-FEB", "2QE-FEB"),
-        ("2BQ", "2BQE"),
-        ("2BQ-FEB", "2BQE-FEB"),
         ("2Y", "2YE"),
         ("2Y-MAR", "2YE-MAR"),
-        ("2BA-MAR", "2BYE-MAR"),
     ],
 )
 def test_resample_frequency_ME_QE_YE_error_message(series_and_frame, freq, freq_depr):
@@ -978,3 +975,22 @@ def test_corner_cases_period(simple_period_range_series):
     # it works
     result = len0pts.resample("Y-DEC").mean()
     assert len(result) == 0
+
+
+@pytest.mark.parametrize(
+    "freq_depr",
+    [
+        "2BME",
+        "2CBME",
+        "2SME",
+        "2BQE-FEB",
+        "2BYE-MAR",
+    ],
+)
+def test_resample_frequency_invalid_freq(series_and_frame, freq_depr):
+    # GH#9586
+    msg = f"Invalid frequency: {freq_depr[1:]}"
+
+    obj = series_and_frame
+    with pytest.raises(ValueError, match=msg):
+        obj.resample(freq_depr)