If only the year operand is given, cal shall produce a calendar for all twelve months in the given calendar year.

cal.c

@@ -146,7 +146,7 @@ main(int argc, char *argv[])
 	fday = 0;
 
 	ncols = 3;
-	nmons = 1;
+	nmons = 0;
 
 	ARGBEGIN {
 	case '1':
@@ -183,6 +183,15 @@ main(int argc, char *argv[])
 		usage();
 	} ARGEND;
 
+	if (nmons == 0) {
+		if (argc == 1) {
+			month = 1;
+			nmons = 12;
+		} else {
+			nmons = 1;
+		}
+	}
+
 	switch (argc) {
 	case 2:
 		month = estrtol(argv[0], 0);