BUG: fixes weekday for dates before 1752 (#53795)

* switch to gauss algo

* precommit cleanup

* missed formatting and whatsnew

* add hypothesis and negative test

* update parametric test

* requested changes

* move whatsnew to 2.0.3

* post-merge fixup

* note source of implementation, simplify, include negative dates in parametric test

---------

Co-authored-by: MarcoGorelli <33491632+MarcoGorelli@users.noreply.github.com>
Co-authored-by: Marco Edward Gorelli <marcogorelli@protonmail.com>

Author: mcgeestocks

doc/source/whatsnew/v2.0.3.rst

@@ -13,6 +13,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
+- Bug in :meth:`Timestamp.weekday`` was returning incorrect results before ``'0000-02-29'`` (:issue:`53738`)
 - Fixed performance regression in merging on datetime-like columns (:issue:`53231`)
 - Fixed regression when :meth:`DataFrame.to_string` creates extra space for string dtypes (:issue:`52690`)
 - For external ExtensionArray implementations, restored the default use of ``_values_for_factorize`` for hashing arrays (:issue:`53475`)

pandas/_libs/tslibs/ccalendar.pyx

@@ -2,7 +2,6 @@
 """
 Cython implementations of functions resembling the stdlib calendar module
 """
-
 cimport cython
 from numpy cimport (
     int32_t,
@@ -19,7 +18,7 @@ cdef int32_t* days_per_month_array = [
     31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
     31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
 
-cdef int* sakamoto_arr = [0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4]
+cdef int* em = [0, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334]
 
 # The first 13 entries give the month days elapsed as of the first of month N
 # (or the total number of days in the year for N=13) in non-leap years.
@@ -76,11 +75,22 @@ cpdef int32_t get_days_in_month(int year, Py_ssize_t month) noexcept nogil:
 
 @cython.wraparound(False)
 @cython.boundscheck(False)
-@cython.cdivision
+@cython.cdivision(True)
+cdef long quot(long a , long b) noexcept nogil:
+    cdef long x
+    x = a/b
+    if (a < 0):
+        x -= (a % b != 0)
+    return x
+
+
+@cython.wraparound(False)
+@cython.boundscheck(False)
+@cython.cdivision(True)
 cdef int dayofweek(int y, int m, int d) noexcept nogil:
     """
     Find the day of week for the date described by the Y/M/D triple y, m, d
-    using Sakamoto's method, from wikipedia.
+    using Gauss' method, from wikipedia.
 
     0 represents Monday.  See [1]_.
 
@@ -103,16 +113,27 @@ cdef int dayofweek(int y, int m, int d) noexcept nogil:
     [1] https://docs.python.org/3/library/calendar.html#calendar.weekday
 
     [2] https://en.wikipedia.org/wiki/\
-    Determination_of_the_day_of_the_week#Sakamoto.27s_methods
+    Determination_of_the_day_of_the_week#Gauss's_algorithm
     """
+    # Note: this particular implementation comes from
+    # http://berndt-schwerdtfeger.de/wp-content/uploads/pdf/cal.pdf
     cdef:
-        int day
-
-    y -= m < 3
-    day = (y + y / 4 - y / 100 + y / 400 + sakamoto_arr[m - 1] + d) % 7
-    # convert to python day
-    return (day + 6) % 7
-
+        long c
+        int g
+        int f
+        int e
+
+    if (m < 3):
+        y -= 1
+
+    c = quot(y, 100)
+    g = y - c * 100
+    f = 5 * (c - quot(c, 4) * 4)
+    e = em[m]
+
+    if (m > 2):
+        e -= 1
+    return (-1 + d + e + f + g + g/4) % 7
 
 cdef bint is_leapyear(int64_t year) noexcept nogil:
     """

pandas/tests/scalar/timestamp/test_timestamp.py

@@ -14,6 +14,10 @@
     tzlocal,
     tzutc,
 )
+from hypothesis import (
+    given,
+    strategies as st,
+)
 import numpy as np
 import pytest
 import pytz
@@ -223,6 +227,39 @@ def test_resolution(self):
         assert dt.as_unit("ms").resolution == Timedelta(milliseconds=1)
         assert dt.as_unit("s").resolution == Timedelta(seconds=1)
 
+    @pytest.mark.parametrize(
+        "date_string, expected",
+        [
+            ("0000-2-29", 1),
+            ("0000-3-1", 2),
+            ("1582-10-14", 3),
+            ("-0040-1-1", 4),
+            ("2023-06-18", 6),
+        ],
+    )
+    def test_dow_historic(self, date_string, expected):
+        # GH 53738
+        ts = Timestamp(date_string)
+        dow = ts.weekday()
+        assert dow == expected
+
+    @given(
+        ts=st.datetimes(),
+        sign=st.sampled_from(["-", ""]),
+    )
+    def test_dow_parametric(self, ts, sign):
+        # GH 53738
+        ts = (
+            f"{sign}{str(ts.year).zfill(4)}"
+            f"-{str(ts.month).zfill(2)}"
+            f"-{str(ts.day).zfill(2)}"
+        )
+        result = Timestamp(ts).weekday()
+        expected = (
+            (np.datetime64(ts) - np.datetime64("1970-01-01")).astype("int64") - 4
+        ) % 7
+        assert result == expected
+
 
 class TestTimestamp:
     def test_default_to_stdlib_utc(self):