diff --git a/src/backend/utils/adt/formatting.c b/src/backend/utils/adt/formatting.c
index 25af8a212b..2aa6df1756 100644
--- a/src/backend/utils/adt/formatting.c
+++ b/src/backend/utils/adt/formatting.c
@@ -412,7 +412,7 @@ typedef struct
 				mi,
 				ss,
 				ssss,
-				d,
+				d,				/* stored as 1-7, Sunday = 1, 0 means missing */
 				dd,
 				ddd,
 				mm,
@@ -2897,6 +2897,7 @@ DCH_from_char(FormatNode *node, char *in, TmFromChar *out)
 				from_char_seq_search(&value, &s, days, ONE_UPPER,
 									 MAX_DAY_LEN, n);
 				from_char_set_int(&out->d, value, n);
+				out->d++;
 				break;
 			case DCH_DY:
 			case DCH_Dy:
@@ -2904,6 +2905,7 @@ DCH_from_char(FormatNode *node, char *in, TmFromChar *out)
 				from_char_seq_search(&value, &s, days, ONE_UPPER,
 									 MAX_DY_LEN, n);
 				from_char_set_int(&out->d, value, n);
+				out->d++;
 				break;
 			case DCH_DDD:
 				from_char_parse_int(&out->ddd, &s, n);
@@ -2919,11 +2921,13 @@ DCH_from_char(FormatNode *node, char *in, TmFromChar *out)
 				break;
 			case DCH_D:
 				from_char_parse_int(&out->d, &s, n);
-				out->d--;
 				s += SKIP_THth(n->suffix);
 				break;
 			case DCH_ID:
 				from_char_parse_int_len(&out->d, &s, 1, n);
+				/* Shift numbering to match Gregorian where Sunday = 1 */
+				if (++out->d > 7)
+					out->d = 1;
 				s += SKIP_THth(n->suffix);
 				break;
 			case DCH_WW:
@@ -3534,7 +3538,7 @@ do_to_timestamp(text *date_txt, text *fmt,
 	if (tmfc.w)
 		tmfc.dd = (tmfc.w - 1) * 7 + 1;
 	if (tmfc.d)
-		tm->tm_wday = tmfc.d;
+		tm->tm_wday = tmfc.d - 1;	/* convert to native numbering */
 	if (tmfc.dd)
 		tm->tm_mday = tmfc.dd;
 	if (tmfc.ddd)
diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index 2adc178de4..50ef8976be 100644
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -3775,18 +3775,22 @@ isoweek2date(int woy, int *year, int *mon, int *mday)
 
 /* isoweekdate2date()
  *
- *	Convert an ISO 8601 week date (ISO year, ISO week and day of week) into a Gregorian date.
+ *	Convert an ISO 8601 week date (ISO year, ISO week) into a Gregorian date.
+ *	Gregorian day of week sent so weekday strings can be supplied.
  *	Populates year, mon, and mday with the correct Gregorian values.
  *	year must be passed in as the ISO year.
  */
 void
-isoweekdate2date(int isoweek, int isowday, int *year, int *mon, int *mday)
+isoweekdate2date(int isoweek, int wday, int *year, int *mon, int *mday)
 {
 	int			jday;
 
 	jday = isoweek2j(*year, isoweek);
-	jday += isowday - 1;
-
+	/* convert Gregorian week start (Sunday=1) to ISO week start (Monday=1) */
+	if (wday > 1)
+		jday += wday - 2;
+	else
+		jday += 6;
 	j2date(jday, year, mon, mday);
 }
 
diff --git a/src/include/utils/timestamp.h b/src/include/utils/timestamp.h
index 665e969498..e7cdb417e5 100644
--- a/src/include/utils/timestamp.h
+++ b/src/include/utils/timestamp.h
@@ -236,7 +236,7 @@ extern int	timestamp_cmp_internal(Timestamp dt1, Timestamp dt2);
 
 extern int	isoweek2j(int year, int week);
 extern void isoweek2date(int woy, int *year, int *mon, int *mday);
-extern void isoweekdate2date(int isoweek, int isowday, int *year, int *mon, int *mday);
+extern void isoweekdate2date(int isoweek, int wday, int *year, int *mon, int *mday);
 extern int	date2isoweek(int year, int mon, int mday);
 extern int	date2isoyear(int year, int mon, int mday);
 extern int	date2isoyearday(int year, int mon, int mday);