Print an array of dates

you can iterate the array:

foreach (Array as $key=> $values) {
    foreach ($values as $value) {
        echo "$value<br>";
    }
}

In this simple way you can print the value of sub-level 2 of the array.

A typical solution is with foreach, assuming that the array is $dias :

foreach($dias as $dia) {

    foreach ($dia as $fecha) {

        echo $fecha . '<br>';

    }

}

Another possibility would be to use the function implode() :

$arrayDays = [

    'Monday' => [ '2017-07-03', '2017-07-17', '2017-07-31' ],
    'Friday' => [ '2017-06-30', '2017-07-14', '2017-07-28' ]
];

foreach( $arrayDays as $date ) {

    echo implode( PHP_EOL, $date ).PHP_EOL;
}

See Demo