diff --git a/usr.bin/sort/bwstring.c b/usr.bin/sort/bwstring.c
--- a/usr.bin/sort/bwstring.c
+++ b/usr.bin/sort/bwstring.c
@@ -43,63 +43,114 @@
 
 bool byte_sort;
 
-static wchar_t **wmonths;
-static char **cmonths;
+struct wmonth {
+	wchar_t *mon;
+	wchar_t *ab;
+	wchar_t *alt;
+};
 
-/* initialise months */
+struct cmonth {
+	char *mon;
+	char *ab;
+	char *alt;
+};
+
+static struct wmonth *wmonths;
+static struct cmonth *cmonths;
+
+static int
+populate_cmonth(char **field, const nl_item item, int idx)
+{
+	char *tmp, *m;
+	size_t i, len;
+
+	tmp = nl_langinfo(item);
+	if (debug_sort)
+		printf("month[%d]=%s\n", idx, tmp);
+	if (*tmp == '\0')
+		return (0);
+	m = sort_strdup(tmp);
+	len = strlen(tmp);
+	for (i = 0; i < len; i++)
+		m[i] = toupper(m[i]);
+	*field = m;
+
+	return (1);
+}
+
+static int
+populate_wmonth(wchar_t **field, const nl_item item, int idx)
+{
+	wchar_t *m;
+	char *tmp;
+	size_t i, len;
+
+	tmp = nl_langinfo(item);
+	if (debug_sort)
+		printf("month[%d]=%s\n", idx, tmp);
+	if (*tmp == '\0')
+		return (0);
+	len = strlen(tmp);
+	m = sort_malloc(SIZEOF_WCHAR_STRING(len + 1));
+	if (mbstowcs(m, tmp, len) == ((size_t) - 1)) {
+		sort_free(m);
+		return (0);
+	}
+	m[len] = L'\0';
+	for (i = 0; i < len; i++)
+		m[i] = towupper(m[i]);
+	*field = m;
+
+	return (1);
+}
 
 void
 initialise_months(void)
 {
-	const nl_item item[12] = { ABMON_1, ABMON_2, ABMON_3, ABMON_4,
+	const nl_item mon_item[12] = { MON_1, MON_2, MON_3, MON_4,
+	    MON_5, MON_6, MON_7, MON_8, MON_9, MON_10,
+	    MON_11, MON_12 };
+	const nl_item ab_item[12] = { ABMON_1, ABMON_2, ABMON_3, ABMON_4,
 	    ABMON_5, ABMON_6, ABMON_7, ABMON_8, ABMON_9, ABMON_10,
 	    ABMON_11, ABMON_12 };
-	char *tmp;
-	size_t len;
-
+	const nl_item alt_item[12] = { ALTMON_1, ALTMON_2, ALTMON_3, ALTMON_4,
+	    ALTMON_5, ALTMON_6, ALTMON_7, ALTMON_8, ALTMON_9, ALTMON_10,
+	    ALTMON_11, ALTMON_12 };
+	int i;
+
+	/*
+	 * Handle all possible month formats: abbrevation, full name,
+	 * standalone name (without case ending).
+	 */
 	if (mb_cur_max == 1) {
 		if (cmonths == NULL) {
-			char *m;
-
-			cmonths = sort_malloc(sizeof(char*) * 12);
-			for (int i = 0; i < 12; i++) {
-				cmonths[i] = NULL;
-				tmp = nl_langinfo(item[i]);
-				if (debug_sort)
-					printf("month[%d]=%s\n", i, tmp);
-				if (*tmp == '\0')
+			cmonths = sort_malloc(sizeof(struct cmonth) * 12);
+			for (i = 0; i < 12; i++) {
+				if (!populate_cmonth(&cmonths[i].mon,
+				    mon_item[i], i))
+					continue;
+				if (!populate_cmonth(&cmonths[i].ab,
+				    ab_item[i], i))
+					continue;
+				if (!populate_cmonth(&cmonths[i].alt,
+				    alt_item[i], i))
 					continue;
-				m = sort_strdup(tmp);
-				len = strlen(tmp);
-				for (unsigned int j = 0; j < len; j++)
-					m[j] = toupper(m[j]);
-				cmonths[i] = m;
 			}
 		}
 
 	} else {
 		if (wmonths == NULL) {
-			wchar_t *m;
-
-			wmonths = sort_malloc(sizeof(wchar_t *) * 12);
-			for (int i = 0; i < 12; i++) {
-				wmonths[i] = NULL;
-				tmp = nl_langinfo(item[i]);
-				if (debug_sort)
-					printf("month[%d]=%s\n", i, tmp);
-				if (*tmp == '\0')
+			wmonths = sort_malloc(sizeof(struct wmonth) * 12);
+			for (i = 0; i < 12; i++) {
+				if (!populate_wmonth(&wmonths[i].mon,
+				    mon_item[i], i))
 					continue;
-				len = strlen(tmp);
-				m = sort_malloc(SIZEOF_WCHAR_STRING(len + 1));
-				if (mbstowcs(m, tmp, len) ==
-				    ((size_t) - 1)) {
-					sort_free(m);
+				if (!populate_wmonth(&wmonths[i].ab,
+				    ab_item[i], i))
+					continue;
+				if (!populate_wmonth(&wmonths[i].alt,
+				    alt_item[i], i))
 					continue;
-				}
-				m[len] = L'\0';
-				for (unsigned int j = 0; j < len; j++)
-					m[j] = towupper(m[j]);
-				wmonths[i] = m;
 			}
 		}
 	}
@@ -754,8 +805,11 @@
 			++s;
 
 		for (int i = 11; i >= 0; --i) {
-			if (cmonths[i] &&
-			    (s == strstr(s, cmonths[i])))
+			if (cmonths[i].mon && (s == strstr(s, cmonths[i].mon)))
+				return (i);
+			if (cmonths[i].ab && (s == strstr(s, cmonths[i].ab)))
+				return (i);
+			if (cmonths[i].alt && (s == strstr(s, cmonths[i].alt)))
 				return (i);
 		}
 
@@ -769,7 +823,11 @@
 			++s;
 
 		for (int i = 11; i >= 0; --i) {
-			if (wmonths[i] && (s == wcsstr(s, wmonths[i])))
+			if (wmonths[i].ab && (s == wcsstr(s, wmonths[i].ab)))
+				return (i);
+			if (wmonths[i].mon && (s == wcsstr(s, wmonths[i].mon)))
+				return (i);
+			if (wmonths[i].alt && (s == wcsstr(s, wmonths[i].alt)))
 				return (i);
 		}
 	}
diff --git a/usr.bin/sort/sort.1.in b/usr.bin/sort/sort.1.in
--- a/usr.bin/sort/sort.1.in
+++ b/usr.bin/sort/sort.1.in
@@ -30,7 +30,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd September 4, 2019
+.Dd November 30, 2023
 .Dt SORT 1
 .Os
 .Sh NAME
@@ -179,7 +179,7 @@
 .It Fl i , Fl Fl ignore-nonprinting
 Ignore all non-printable characters.
 .It Fl M , Fl Fl month-sort , Fl Fl sort=month
-Sort by month abbreviations.
+Sort by month.
 Unknown strings are considered smaller than the month names.
 .It Fl n , Fl Fl numeric-sort , Fl Fl sort=numeric
 Sort fields numerically by arithmetic value.
diff --git a/usr.bin/sort/tests/Makefile b/usr.bin/sort/tests/Makefile
--- a/usr.bin/sort/tests/Makefile
+++ b/usr.bin/sort/tests/Makefile
@@ -2,6 +2,7 @@
 PACKAGE=	tests
 
 NETBSD_ATF_TESTS_SH=	sort_test
+ATF_TESTS_SH=		sort_monthsort_test
 
 ${PACKAGE}FILES+=		d_any_char_dflag_out.txt
 ${PACKAGE}FILES+=		d_any_char_fflag_out.txt
diff --git a/usr.bin/sort/tests/sort_monthsort_test.sh b/usr.bin/sort/tests/sort_monthsort_test.sh
new file mode 100755
--- /dev/null
+++ b/usr.bin/sort/tests/sort_monthsort_test.sh
@@ -0,0 +1,159 @@
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+# Copyright (c) 2023 Christos Margiolis <christos@FreeBSD.org>
+#
+
+get_months_fmt()
+{
+	rm -f in
+        for i in $(seq 12 1); do
+                printf "2000-%02d-01\n" ${i} | xargs -I{} \
+                date -jf "%Y-%m-%d" {} "${1}" >>in
+        done
+}
+
+atf_test_case monthsort_english
+monthsort_english_head()
+{
+	atf_set "descr" "Test the -M flag with English months"
+}
+monthsort_english_body()
+{
+	export LC_TIME="en_US.UTF-8"
+
+	cat >expout <<EOF
+January
+February
+March
+April
+May
+June
+July
+August
+September
+October
+November
+December
+EOF
+
+	# No need to test the rest of the formats (%b and %OB) as %b is a
+	# substring of %B and %OB is the same as %B.
+	get_months_fmt '+%B'
+	atf_check -o file:expout sort -M in
+}
+
+atf_test_case monthsort_all_formats_greek
+monthsort_all_formats_greek_head()
+{
+	atf_set "descr" "Test the -M flag with all possible Greek month formats"
+}
+monthsort_all_formats_greek_body()
+{
+	# Test with the Greek locale, since, unlike English, the
+	# abbreviation/full-name and standalone formats are different.
+	export LC_TIME="el_GR.UTF-8"
+
+	# Abbreviation format (e.g Jan, Ιαν)
+	cat >expout <<EOF
+Ιαν
+Φεβ
+Μαρ
+Απρ
+Μαΐ
+Ιουν
+Ιουλ
+Αυγ
+Σεπ
+Οκτ
+Νοε
+Δεκ
+EOF
+	get_months_fmt '+%b'
+	atf_check -o file:expout sort -M in
+
+	# Full-name format (e.g January, Ιανουαρίου)
+	cat >expout <<EOF
+Ιανουαρίου
+Φεβρουαρίου
+Μαρτίου
+Απριλίου
+Μαΐου
+Ιουνίου
+Ιουλίου
+Αυγούστου
+Σεπτεμβρίου
+Οκτωβρίου
+Νοεμβρίου
+Δεκεμβρίου
+EOF
+	get_months_fmt '+%B'
+	atf_check -o file:expout sort -M in
+
+	# Standalone format (e.g January, Ιανουάριος)
+	cat >expout <<EOF
+Ιανουάριος
+Φεβρουάριος
+Μάρτιος
+Απρίλιος
+Μάϊος
+Ιούνιος
+Ιούλιος
+Αύγουστος
+Σεπτέμβριος
+Οκτώβριος
+Νοέμβριος
+Δεκέμβριος
+EOF
+	get_months_fmt '+%OB'
+	atf_check -o file:expout sort -M in
+}
+
+atf_test_case monthsort_mixed_formats_greek
+monthsort_mixed_formats_greek_head()
+{
+	atf_set "descr" "Test the -M flag with mixed Greek month formats"
+}
+monthsort_mixed_formats_greek_body()
+{
+	export LC_TIME="el_GR.UTF-8"
+
+	cat >in <<EOF
+Δεκέμβριος
+Νοεμβρίου
+Οκτ
+Σεπ
+Αυγ
+Ιούλιος
+Ιουνίου
+Μαΐου
+Απριλίου
+Μάρτιος
+Φεβρουάριος
+Ιανουάριος
+EOF
+
+	cat >expout <<EOF
+Ιανουάριος
+Φεβρουάριος
+Μάρτιος
+Απριλίου
+Μαΐου
+Ιουνίου
+Ιούλιος
+Αυγ
+Σεπ
+Οκτ
+Νοεμβρίου
+Δεκέμβριος
+EOF
+
+	atf_check -o file:expout sort -M in
+}
+
+atf_init_test_cases()
+{
+	atf_add_test_case monthsort_english
+	atf_add_test_case monthsort_all_formats_greek
+	atf_add_test_case monthsort_mixed_formats_greek
+}