WordPress transients for a shortcode

I have a small shortcode which basically takes data from a JSON file and displays it on a page. The data in JSON is updated weekly, how can I use a transient so the data for the current week is cached?

Here is my shortcode

function week_agenda() {
        $days = json_decode(file_get_contents('json_file'));
        unset($days[0]);
        ob_start();
    ?>
    <div class="table-responsive">
        <table class="table">
            <thead>
                <tr>
                    <th> Title </th>
                    <th>Content</th>
                </tr>
            </thead>
            <tbody>
                <?php foreach($days as $data) { ?>
                <tr>
                    <td><?php echo $data[0]; ?></td>
                </tr>
                <?php } ?>
            </tbody>
        </table>
    </div>
    <?php
    $output = ob_get_clean();
    return $output;
}

Use this instead of the line in wich you define $days (your second line):

$transient = get_transient( 'your_transient_key' );

if( !$transient ):

    $days = file_get_contents( 'json_file' );

    set_transient( 'your_transient_key', $days, DAY_IN_SECONDS*7 );


else:

    $days = $transient;

endif;

$days = json_decode( $days );

... 

May be a bit rough but you get the idea.