<div class="calender">
    <div class='month_name' style='background-color:#9d61bf; color:#ffffff;'>
	<?php echo 'Oct'//month_de($thisMonth); ?></div>
    <div class='day_box'>Mo</div>
	<div class='day_box'>Di</div>
	<div class='day_box'>Mi</div>
	<div class='day_box'>Do</div>
	<div class='day_box'>Fr</div>
	<div class='day_box' style='color:#b51b1e;'>Sa</div>
	<div class='day_box' style='color:#b51b1e;'>So</div>
    <?php

//$daten = get_Kalenderdaten("kalenderdaten");
//$daten = file_get_contents("cal.txt");
//$content = preg_split('/;/',$daten);

// you should just use file() to read the lines of the file

// cal2.txt is my test file with dates in YYYY-MM-DD format and no ; on the end of each line
$content = file('cal2.txt',FILE_IGNORE_NEW_LINES);

// loop over the data once, building an array with the date ranges expanded
// the entries in the data array are -
// BS (booking start), B (booked), BE (booking end), SS (closed start), S (closed), SE (closed end)
$data = [];
foreach($content as $row)
{
						// use explode() for simple cases
	list($typ, $beginn, $ende) = explode(',',$row);

	// produce the date range
	$begin = new DateTime($beginn);
	$end = new DateTime($ende);
	// include the end date in the interval
	$end = $end->modify('+1 day');
	$interval = DateInterval::createFromDateString('1 day');
	$period = new DatePeriod($begin, $interval, $end);

	// loop over the date range
	foreach($period as $dt)
	{
		$date = $dt->format("Y-m-d");
		$extra = '';
		// start
		if($date == $beginn) $extra = 'S';
		// end
		if($date == $ende) $extra = 'E';
		$data[$date][] = $typ.$extra;
	}
}

// loop to produce the output
foreach(range(1,$thisMonthLastDay) as $d)
{
	// day name
	$tag_eng = date('D', mktime(0,0,0,date("m"),$d,date('Y')));
	// current date
	$datum = date('Y-m-d', mktime(0,0,0,date("m"),$d,date('Y')));
	$colvar ="";

	// determine the combined value for the current date
	$val = implode('-',$data[$datum]??[]);

	// map the value to the $colvar value
	switch($val)
	{
		case 'BS':
		$colvar = " box-bg-wr";
		break;
		case 'B':
		$colvar = " box-bg-r";
		break;
		case 'BE':
		$colvar = " box-bg-rw";
		break;

		case 'SS':
		$colvar = " box-bg-wg";
		break;
		case 'S':
		$colvar = " box-bg-g";
		break;
		case 'SE':
		$colvar = " box-bg-gw";
		break;

		case 'BE-SS':
		$colvar = " box-bg-rwg";
		break;
		case 'SE-BS':
		$colvar = " box-bg-gwr";
		break;

		case 'SE-SS':
		$colvar = " box-bg-g"; // the OP's logic produces " box-bg-wg" for this case, based on the SS match, and never reaches the Schließung Ende und Start am selben Tag logic to produce the " box-bg-g"
		break;

		case 'BE-BS':
		$colvar = " box-bg-rwr";
		break;
	}

	$col = columnStart($tag_eng);
	echo "<div class='day_box{$colvar}' id='$datum'
		style='grid-column-start:$col;'>$d</div>\n";
}
?>
    <div style="position: relative; left: -120px; top: 40px; font-size: 1rem;">
        <div class='day_box' style='background-color: #f17571; color: #ffffff; width: 15px;
			height: 15px;'></div><span style="position: relative; bottom: 22px; left: 20px;">
			reserviert</span>
    </div>
</div>