Reject nonpositive weekday occurrences

src/pendulum/date.py

@@ -550,13 +550,15 @@ def nth_of(self, unit: str, nth: int, day_of_week: WeekDay) -> Self:
         Supported units are month, quarter and year.
 
         :param unit: The unit to use
-        :param nth: The occurrence to use
+        :param nth: The occurrence to use, starting at 1
         :param day_of_week: The day of week to set to.
         """
         if unit not in ["month", "quarter", "year"]:
             raise ValueError(f'Invalid unit "{unit}" for first_of()')
 
-        dt = cast("Self", getattr(self, f"_nth_of_{unit}")(nth, day_of_week))
+        dt = None
+        if nth > 0:
+            dt = cast("Self", getattr(self, f"_nth_of_{unit}")(nth, day_of_week))
         if not dt:
             raise PendulumException(
                 f"Unable to find occurrence {nth}"

src/pendulum/datetime.py

@@ -1005,11 +1005,14 @@ def nth_of(self, unit: str, nth: int, day_of_week: WeekDay) -> Self:
         to indicate the desired day_of_week, ex. DateTime.MONDAY.
 
         Supported units are month, quarter and year.
+        Occurrences start at 1.
         """
         if unit not in ["month", "quarter", "year"]:
             raise ValueError(f'Invalid unit "{unit}" for first_of()')
 
-        dt = cast("Self | None", getattr(self, f"_nth_of_{unit}")(nth, day_of_week))
+        dt = None
+        if nth > 0:
+            dt = cast("Self | None", getattr(self, f"_nth_of_{unit}")(nth, day_of_week))
         if not dt:
             raise PendulumException(
                 f"Unable to find occurrence {nth}"

tests/date/test_day_of_week_modifiers.py

@@ -196,6 +196,16 @@ def test_nth_of_month_outside_scope():
         d.nth_of("month", 6, pendulum.MONDAY)
 
 
+@pytest.mark.parametrize("date_type", [pendulum.Date, pendulum.DateTime])
+@pytest.mark.parametrize("unit", ["month", "quarter", "year"])
+@pytest.mark.parametrize("nth", [0, -1, -4])
+def test_nth_of_nonpositive_occurrence(date_type, unit, nth):
+    d = date_type(2024, 5, 15)
+
+    with pytest.raises(PendulumException):
+        d.nth_of(unit, nth, pendulum.FRIDAY)
+
+
 def test_nth_of_month_outside_year():
     d = pendulum.date(1975, 12, 5)