diff --git a/doc/api/next_api_changes/deprecations.rst b/doc/api/next_api_changes/deprecations.rst
@@ -284,3 +284,7 @@ is deprecated, set the offset to 0 instead.
 ``RendererCairo.fontweights``, ``RendererCairo.fontangles``
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 ... are deprecated.
+
+``autofmt_xdate(which=None)``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This is deprecated, use its more explicit synonym, ``which="major"``, instead.
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -574,7 +574,8 @@ def get_constrained_layout_pads(self, relative=False):
 
         return w_pad, h_pad, wspace, hspace
 
-    def autofmt_xdate(self, bottom=0.2, rotation=30, ha='right', which=None):
+    def autofmt_xdate(
+            self, bottom=0.2, rotation=30, ha='right', which='major'):
         """
         Date ticklabels often overlap, so it is useful to rotate them
         and right align them.  Also, a common use case is a number of
@@ -586,18 +587,19 @@ def autofmt_xdate(self, bottom=0.2, rotation=30, ha='right', which=None):
         Parameters
         ----------
         bottom : scalar
-            The bottom of the subplots for :meth:`subplots_adjust`.
-
+            The bottom of the subplots for `subplots_adjust`.
         rotation : angle in degrees
             The rotation of the xtick labels.
-
         ha : str
             The horizontal alignment of the xticklabels.
-
-        which : {None, 'major', 'minor', 'both'}
-            Selects which ticklabels to rotate. Default is None which works
-            the same as major.
+        which : {'major', 'minor', 'both'}, default: 'major'
+            Selects which ticklabels to rotate.
         """
+        if which is None:
+            cbook.warn_deprecated(
+                "3.3", message="Support for passing which=None to mean "
+                "which='major' is deprecated since %(since)s and will be "
+                "removed %(removal)s.")
         allsubplots = all(hasattr(ax, 'is_last_row') for ax in self.axes)
         if len(self.axes) == 1:
             for label in self.axes[0].get_xticklabels(which=which):

diff --git a/lib/matplotlib/tests/test_figure.py b/lib/matplotlib/tests/test_figure.py
@@ -2,6 +2,10 @@
 from pathlib import Path
 import platform
 import warnings
+try:
+    from contextlib import nullcontext
+except ImportError:
+    from contextlib import ExitStack as nullcontext  # Py3.6
 
 import matplotlib as mpl
 from matplotlib import rcParams
@@ -318,7 +322,9 @@ def test_autofmt_xdate(which):
             'FixedFormatter should only be used together with FixedLocator')
         ax.xaxis.set_minor_formatter(FixedFormatter(minors))
 
-    fig.autofmt_xdate(0.2, angle, 'right', which)
+    with (pytest.warns(mpl.MatplotlibDeprecationWarning) if which is None else
+          nullcontext()):
+        fig.autofmt_xdate(0.2, angle, 'right', which)
 
     if which in ('both', 'major', None):
         for label in fig.axes[0].get_xticklabels(False, 'major'):