diff --git a/pandas/_libs/tslibs/offsets.pyx b/pandas/_libs/tslibs/offsets.pyx
index fb07e3fe7547e..fa5ef0e7adf59 100644
--- a/pandas/_libs/tslibs/offsets.pyx
+++ b/pandas/_libs/tslibs/offsets.pyx
@@ -598,7 +598,7 @@ cdef class BaseOffset:
 
     def _get_offset_day(self, other: datetime) -> int:
         # subclass must implement `_day_opt`; calling from the base class
-        # will raise NotImplementedError.
+        # will implicitly assume day_opt = "business_end", see get_day_of_month.
         cdef:
             npy_datetimestruct dts
         pydate_to_dtstruct(other, &dts)
@@ -3651,7 +3651,6 @@ def shift_months(const int64_t[:] dtindex, int months, object day_opt=None):
                 out[i] = dtstruct_to_dt64(&dts)
     elif day_opt in ["start", "end", "business_start", "business_end"]:
         _shift_months(dtindex, out, count, months, day_opt)
-
     else:
         raise ValueError("day must be None, 'start', 'end', "
                          "'business_start', or 'business_end'")
@@ -3841,7 +3840,7 @@ def shift_month(stamp: datetime, months: int, day_opt: object=None) -> datetime:
     return stamp.replace(year=year, month=month, day=day)
 
 
-cdef inline int get_day_of_month(npy_datetimestruct* dts, day_opt) nogil except? -1:
+cdef inline int get_day_of_month(npy_datetimestruct* dts, str day_opt) nogil:
     """
     Find the day in `other`'s month that satisfies a DateOffset's is_on_offset
     policy, as described by the `day_opt` argument.
@@ -3867,27 +3866,23 @@ cdef inline int get_day_of_month(npy_datetimestruct* dts, day_opt) nogil except?
     >>> get_day_of_month(other, 'end')
     30
 
+    Notes
+    -----
+    Caller is responsible for ensuring one of the four accepted day_opt values
+    is passed.
     """
-    cdef:
-        int days_in_month
 
     if day_opt == "start":
         return 1
     elif day_opt == "end":
-        days_in_month = get_days_in_month(dts.year, dts.month)
-        return days_in_month
+        return get_days_in_month(dts.year, dts.month)
     elif day_opt == "business_start":
         # first business day of month
         return get_firstbday(dts.year, dts.month)
-    elif day_opt == "business_end":
+    else:
+        # i.e. day_opt == "business_end":
         # last business day of month
         return get_lastbday(dts.year, dts.month)
-    elif day_opt is not None:
-        raise ValueError(day_opt)
-    elif day_opt is None:
-        # Note: unlike `shift_month`, get_day_of_month does not
-        # allow day_opt = None
-        raise NotImplementedError
 
 
 cpdef int roll_convention(int other, int n, int compare) nogil:
@@ -3941,6 +3936,10 @@ def roll_qtrday(other: datetime, n: int, month: int,
     cdef:
         int months_since
         npy_datetimestruct dts
+
+    if day_opt not in ["start", "end", "business_start", "business_end"]:
+        raise ValueError(day_opt)
+
     pydate_to_dtstruct(other, &dts)
 
     if modby == 12:
diff --git a/pandas/tests/tseries/offsets/test_offsets.py b/pandas/tests/tseries/offsets/test_offsets.py
index 784c04f225630..cffaa7b43d0cf 100644
--- a/pandas/tests/tseries/offsets/test_offsets.py
+++ b/pandas/tests/tseries/offsets/test_offsets.py
@@ -4310,12 +4310,6 @@ def test_all_offset_classes(self, tup):
 
 
 # ---------------------------------------------------------------------
-def test_get_offset_day_error():
-    # subclass of _BaseOffset must override _day_opt attribute, or we should
-    # get a NotImplementedError
-
-    with pytest.raises(NotImplementedError):
-        DateOffset()._get_offset_day(datetime.now())
 
 
 def test_valid_default_arguments(offset_types):