MYSQL，插入工程但它不会连接

So, I am making a kalender sorta thing for school, and im trying to insert multiple rows thingys in the table, The delete function works good but when i try to do the add function it wont work correctly. This is my code =

<?php
$connection = new mysqli('localhost','root','','calendar');

$Days = $_POST['Day'];
$Months = $_POST['Month'];
$Years = $_POST['Year'];
$Names = $_POST['Name'];

$sql = "INSERT INTO birthdays (day), (month), (year), (person) VALUES     ('$Days'), ('$Months'), ('$Years'). ('$Names')";

echo $sql;

$connection->query($sql);

//header('location:index.php');
?>

The outcome:

$sql = "INSERT INTO birthdays (day,month,year,person) VALUES('$Days'), ('$Months'), ('$Years'). ('$Names')";

So it should work fine, But it doesnt save to the table. Im also not that much known with MYSQL yet, I think i did something wrong with the inserting part where i did multiple rows at once, but not sure how to fix it.

(Well, The dates, Names etc he gets on the last page, but that works fine)

The correct SQL syntax is this one:

$sql = "INSERT INTO birthdays (day, month, year, person) VALUES ($Days, $Months, $Years, '$Names')";

but you should use prepared statements with placeholders, something like this:

$pst = $mysqli->prepare("INSERT INTO birthdays (day, month, year, person) VALUES (?,?,?,?)");

$pst->bind_param('iiis', $day, $month, $year,  $name);
$pst->execute();