在Mysql中，我們可以使用WEEKDAY函數來查詢某一天是星期幾。

SELECT WEEKDAY('2021-09-03');

該查詢的結果是：

+------------------------+
| WEEKDAY('2021-09-03') |
+------------------------+
|                      4 |
+------------------------+

其中，0表示星期一，1表示星期二，以此類推。

如果我們想查詢某個月的所有日期是星期幾，可以使用如下查詢：

SELECT
DATE_FORMAT(cal_date, '%Y-%m-%d') AS date,
WEEKDAY(cal_date) AS weekday
FROM
(
SELECT
ADDDATE('2021-09-01', t4 * 10000 + t3 * 1000 + t2 * 100 + t1 * 10 + t0) AS cal_date
FROM
(SELECT 0 t0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t0,
(SELECT 0 t1 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t1,
(SELECT 0 t2 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t2,
(SELECT 0 t3 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t3,
(SELECT 0 t4 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3) t4
) dates
WHERE
MONTH(cal_date) = 9;

該查詢會返回9月份的所有日期及其對應的星期幾。

我們可以將查詢結果使用PHP的循環來輸出：

$query = "SELECT
DATE_FORMAT(cal_date, '%Y-%m-%d') AS date,
WEEKDAY(cal_date) AS weekday
FROM
(
SELECT
ADDDATE('2021-09-01', t4 * 10000 + t3 * 1000 + t2 * 100 + t1 * 10 + t0) AS cal_date
FROM
(SELECT 0 t0 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t0,
(SELECT 0 t1 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t1,
(SELECT 0 t2 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t2,
(SELECT 0 t3 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9) t3,
(SELECT 0 t4 UNION SELECT 1 UNION SELECT 2 UNION SELECT 3) t4
) dates
WHERE
MONTH(cal_date) = 9";
$result = mysqli_query($connection, $query);
while ($row = mysqli_fetch_assoc($result)) {
echo "日期：" . $row['date'] . " 星期：" . ($row['weekday'] + 1) . "
";
}

這樣就可以輸出所有日期及其對應的星期了。