^(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)(?:,(?!\1|\2)(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)){0,6}$
正規表現が絶対的な要件である場合はこれを使用できますが、Martijn の回答をお勧めします。はるかに柔軟で読みやすいです。
PHPでこれをテストした方法は次のとおりです。
<?php
$subject1 = "Mon,Mon";
$subject2 = "Sun,Mon,Fri,Sun";
$subject3 = "Sat";
$subject4 = "Mon,Wed,Tues,Fri,Wed";
$subject5 = "Mon,Tues";
$pattern = '/^(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)(?:,(?!\1|\2)(Mon|Tues|Wed|Thurs|Fri|Sat|Sun)){0,6}$/i';
print_r(preg_match($pattern, $subject1, $matches) . " " . $subject1 . "\n");
print_r(preg_match($pattern, $subject2, $matches) . " " . $subject2 . "\n");
print_r(preg_match($pattern, $subject3, $matches) . " " . $subject3 . "\n");
print_r(preg_match($pattern, $subject4, $matches) . " " . $subject4 . "\n");
print_r(preg_match($pattern, $subject5, $matches) . " " . $subject5 . "\n");
?>
これは以下を出力します:
0 Mon,Mon
0 Sun,Mon,Fri,Sun
1 Sat
1 Mon,Wed,Tues,Fri,Wed
1 Mon,Tues