Series datetime is_month_start (#8844)

Addressing feature request #8678. `is_month_start` returns a series indicating whether each date is the first day of the month.

Authors:
  - Sarah Yurick (https://github.com/sarahyurick)

Approvers:
  - Ram (Ramakrishna Prabhu) (https://github.com/rgsl888prabhu)
  - https://github.com/brandon-b-miller

URL: #8844

python/cudf/cudf/core/series.py

@@ -6446,6 +6446,18 @@ def is_leap_year(self):
             name=self.series.name,
         )
 
+    @property
+    def is_month_start(self):
+        """
+        Boolean indicator if the date is the first day of the month.
+
+        Returns
+        -------
+        Series
+        Booleans indicating if dates are the first day of the month.
+        """
+        return (self.day == 1).fillna(False)
+
     def _get_dt_field(self, field):
         out_column = self.series._column.get_dt_field(field)
         return Series(

python/cudf/cudf/tests/test_datetime.py

@@ -1297,3 +1297,32 @@ def test_is_leap_year():
     got2 = gIndex.is_leap_year
 
     assert_eq(expect2, got2)
+
+
+@pytest.mark.parametrize(
+    "data",
+    [
+        [
+            "2020-05-31",
+            None,
+            "1999-12-01",
+            "2000-12-21",
+            None,
+            "1900-02-28",
+            "1800-03-14",
+            "2100-03-10",
+            "1970-01-01",
+            "1969-12-11",
+        ]
+    ],
+)
+@pytest.mark.parametrize("dtype", ["datetime64[ns]"])
+def test_is_month_start(data, dtype):
+    # Series
+    ps = pd.Series(data, dtype=dtype)
+    gs = cudf.from_pandas(ps)
+
+    expect = ps.dt.is_month_start
+    got = gs.dt.is_month_start
+
+    assert_eq(expect, got)