diff --git a/doc/source/whatsnew/v3.0.4.rst b/doc/source/whatsnew/v3.0.4.rst
index 90e6a5f03f736..a85feb82a070b 100644
--- a/doc/source/whatsnew/v3.0.4.rst
+++ b/doc/source/whatsnew/v3.0.4.rst
@@ -24,6 +24,7 @@ Bug fixes
 
 - Bug in :meth:`DataFrame.iloc` silently ignoring the assignment when setting values with an unordered or duplicated column indexer on a :class:`DataFrame` whose values are referenced by another object (:issue:`65446`)
 - Bug in :meth:`Series.str.__getitem__` raising ``AttributeError`` when underlying array is :class:`ArrowExtensionArray` (:issue:`65112`)
+- Bug in arithmetic adding or subtracting a non-tick :class:`DateOffset` (e.g. :class:`offsets.MonthEnd`, :class:`offsets.QuarterEnd`) to datetime data that could cause a segmentation fault when another thread was running concurrently, e.g. under ``pytest-xdist`` (:issue:`66031`)
 
 
 .. ---------------------------------------------------------------------------
diff --git a/pandas/_libs/tslibs/offsets.pyx b/pandas/_libs/tslibs/offsets.pyx
index 6dcdf0759570c..7873c501b1651 100644
--- a/pandas/_libs/tslibs/offsets.pyx
+++ b/pandas/_libs/tslibs/offsets.pyx
@@ -94,6 +94,31 @@ from .timestamps cimport _Timestamp
 
 from .timestamps import Timestamp
 
+# day_opt enum: avoids touching Python str objects inside nogil loops
+
+cdef enum _DayOpt:
+    DAY_OPT_START = 0
+    DAY_OPT_END = 1
+    DAY_OPT_BUSINESS_START = 2
+    DAY_OPT_BUSINESS_END = 3
+
+
+cdef _DayOpt _str_to_day_opt(str day_opt) except? DAY_OPT_START:
+    if day_opt == "start":
+        return DAY_OPT_START
+    elif day_opt == "end":
+        return DAY_OPT_END
+    elif day_opt == "business_start":
+        return DAY_OPT_BUSINESS_START
+    elif day_opt == "business_end":
+        return DAY_OPT_BUSINESS_END
+    else:
+        raise ValueError(
+            "day must be 'start', 'end', 'business_start', or "
+            f"'business_end', got {day_opt}"
+        )
+
+
 # ---------------------------------------------------------------------
 # Misc Helpers
 
@@ -833,7 +858,7 @@ cdef class BaseOffset:
         cdef:
             npy_datetimestruct dts
         pydate_to_dtstruct(other, &dts)
-        return get_day_of_month(&dts, self._day_opt)
+        return get_day_of_month(&dts, _str_to_day_opt(self._day_opt))
 
     def is_on_offset(self, dt: datetime) -> bool:
         """
@@ -3175,7 +3200,7 @@ cdef class YearOffset(SingleConstructorOffset):
             npy_datetimestruct dts
         pydate_to_dtstruct(other, &dts)
         dts.month = self._month
-        return get_day_of_month(&dts, self._day_opt)
+        return get_day_of_month(&dts, _str_to_day_opt(self._day_opt))
 
     @apply_wraps
     def _apply(self, other: datetime) -> datetime:
@@ -6468,6 +6493,7 @@ cdef ndarray shift_quarters(
         int months_since, n
         npy_datetimestruct dts
         cnp.broadcast mi = cnp.PyArray_MultiIterNew2(out, dtindex)
+        _DayOpt day_opt_enum = _str_to_day_opt(day_opt)
 
     with nogil:
         for i in range(count):
@@ -6481,11 +6507,11 @@ cdef ndarray shift_quarters(
                 n = quarters
 
                 months_since = (dts.month - q1start_month) % modby
-                n = _roll_qtrday(&dts, n, months_since, day_opt)
+                n = _roll_qtrday(&dts, n, months_since, day_opt_enum)
 
                 dts.year = year_add_months(dts, modby * n - months_since)
                 dts.month = month_add_months(dts, modby * n - months_since)
-                dts.day = get_day_of_month(&dts, day_opt)
+                dts.day = get_day_of_month(&dts, day_opt_enum)
 
                 res_val = npy_datetimestruct_to_datetime(reso, &dts)
 
@@ -6521,6 +6547,7 @@ def shift_months(
         ndarray out = cnp.PyArray_EMPTY(dtindex.ndim, dtindex.shape, cnp.NPY_INT64, 0)
         int months_to_roll
         int64_t val, res_val
+        _DayOpt day_opt_enum
 
         cnp.broadcast mi = cnp.PyArray_MultiIterNew2(out, dtindex)
 
@@ -6553,6 +6580,7 @@ def shift_months(
                 cnp.PyArray_MultiIter_NEXT(mi)
 
     else:
+        day_opt_enum = _str_to_day_opt(day_opt)
         with nogil:
             for i in range(count):
 
@@ -6565,11 +6593,13 @@ def shift_months(
                     pandas_datetime_to_datetimestruct(val, reso, &dts)
                     months_to_roll = months
 
-                    months_to_roll = _roll_qtrday(&dts, months_to_roll, 0, day_opt)
+                    months_to_roll = _roll_qtrday(
+                        &dts, months_to_roll, 0, day_opt_enum
+                    )
 
                     dts.year = year_add_months(dts, months_to_roll)
                     dts.month = month_add_months(dts, months_to_roll)
-                    dts.day = get_day_of_month(&dts, day_opt)
+                    dts.day = get_day_of_month(&dts, day_opt_enum)
 
                     res_val = npy_datetimestruct_to_datetime(reso, &dts)
 
@@ -6650,7 +6680,7 @@ def shift_month(stamp: datetime, months: int, day_opt: object = None) -> datetim
     return stamp.replace(year=year, month=month, day=day)
 
 
-cdef int get_day_of_month(npy_datetimestruct* dts, str day_opt) noexcept nogil:
+cdef int get_day_of_month(npy_datetimestruct* dts, _DayOpt day_opt) noexcept nogil:
     """
     Find the day in `other`'s month that satisfies a DateOffset's is_on_offset
     policy, as described by the `day_opt` argument.
@@ -6658,39 +6688,31 @@ cdef int get_day_of_month(npy_datetimestruct* dts, str day_opt) noexcept nogil:
     Parameters
     ----------
     dts : npy_datetimestruct*
-    day_opt : {'start', 'end', 'business_start', 'business_end'}
-        'start': returns 1
-        'end': returns last day of the month
-        'business_start': returns the first business day of the month
-        'business_end': returns the last business day of the month
+    day_opt : _DayOpt enum value
+        DAY_OPT_START: returns 1
+        DAY_OPT_END: returns last day of the month
+        DAY_OPT_BUSINESS_START: returns the first business day of the month
+        DAY_OPT_BUSINESS_END: returns the last business day of the month
 
     Returns
     -------
     day_of_month : int
 
-    Examples
-    -------
-    >>> other = datetime(2017, 11, 14)
-    >>> get_day_of_month(other, 'start')
-    1
-    >>> get_day_of_month(other, 'end')
-    30
-
     Notes
     -----
     Caller is responsible for ensuring one of the four accepted day_opt values
     is passed.
     """
 
-    if day_opt == "start":
+    if day_opt == DAY_OPT_START:
         return 1
-    elif day_opt == "end":
+    elif day_opt == DAY_OPT_END:
         return get_days_in_month(dts.year, dts.month)
-    elif day_opt == "business_start":
+    elif day_opt == DAY_OPT_BUSINESS_START:
         # first business day of month
         return get_firstbday(dts.year, dts.month)
     else:
-        # i.e. day_opt == "business_end":
+        # i.e. day_opt == DAY_OPT_BUSINESS_END:
         # last business day of month
         return get_lastbday(dts.year, dts.month)
 
@@ -6758,13 +6780,13 @@ def roll_qtrday(other: datetime, n: int, month: int,
     else:
         months_since = other.month % modby - month % modby
 
-    return _roll_qtrday(&dts, n, months_since, day_opt)
+    return _roll_qtrday(&dts, n, months_since, _str_to_day_opt(day_opt))
 
 
 cdef int _roll_qtrday(npy_datetimestruct* dts,
                       int n,
                       int months_since,
-                      str day_opt) except? -1 nogil:
+                      _DayOpt day_opt) noexcept nogil:
     """
     See roll_qtrday.__doc__
     """