CPT EVENT - listing by month and filtering by category and month - form select option

Question

I am creating custom post type eventlist filter form. There is two options: category and months. Following this advice List events by month I got display event categories by month. Meaning...

January

• Event post 1
• Event post 2

February

• Event post 1
• Event post 2

etc.

Category choosing works smoothly. Problem comes up when user choose month. By default events are listed by month. How can I reorder events using form options? Is it possible solve using meta_query compare or do I need some other solution?

<form method="get" action="<?php the_permalink();?>" name="tapahtumalista">
<select class="form-select" name="kat">

<?php 
$terms = get_terms( array(
    'taxonomy'      => 'kategori_tapahtumat',
));


// SELECT OPTION CATEGORY
?>

    <option value="">Valitse kategoria</option>
    <?php 
    
    foreach ($terms as $term) { ?>
        <option value="<?php echo $term->slug; ?>" <?php if(isset($_GET['kat']) && $_GET['kat'] == $term->slug) { echo 'selected="selected"'; }?>>
        <?php echo $term->name; ?></option>
    <?php } ?>
</select>

<?php  // SELECT OPTION MONTH ?>
<select class="form-select" name="kk">
    <option value="">Valitse kuukausi</option>
    <?php 
    for ($i = 1; $i <= 12; $i++){
    $month_name = date_i18n('F', mktime(0, 0, 0, $i, 1, 2011)); 
    $month = !empty( $_GET['kk'] ) ? $_GET['kk'] : 0;
    $selected = $month_name == $month;
    ?>
    <option value="<?php echo $month_name; ?>" 
    <?php if(isset($_GET['kk']) && !empty($_GET['kk']) == $selected) { echo 'selected="selected"'; }?>
    <?php $selected ?>> 
        <?php echo $month_name; ?>
    </option>
    
    <?php }?>
</select>
<input type="submit" name="" value="Valitse">

// Selected category from form option 
if($_GET['kat'] && !empty($_GET['kat'])) {
    $selected_cat = $_GET['kat'];
}

//Selected month from form option
if($_GET['kk'] && !empty($_GET['kk'])) {
    $selected_month = $_GET['kk'];  
}


$today = date('Ymd');
$m = get_the_time('m');

$args =  [
    'post_type'         => 'tapahtumat',
    'post_status'       => 'publish',
    'posts_per_page'    => - 1,
    'meta_key'          => 'aloituspaiva', //event start date
    'meta_value'        => $today,
    'orderby'           => 'meta_value',
    'value'             => $today,
    'meta_compare'      => '>=',
    'type'              => 'DATE',
    'order'             => 'ASC',
    'tax_query'         => [
        [
            'taxonomy'  => 'kategori_tapahtumat',
            'field'     => 'slug',
            'terms'     => $selected_cat, //category get from form 
        ]
    ],
    'meta_query'        => [
        'relation'      => 'OR',
        [
            
            'month'     => $m,
            'value'     => $selected_month, // get from form possible?
            'compare'   => '=',
        ]

    ]
];
$query = new WP_Query($args);

// List events by month (default view) - using dboris solution

$all_events = [];

if($query->have_posts()) :
    while ($query->have_posts() ) : $query->the_post();
        $date = strtotime(get_post_meta( get_the_ID(), 'aloituspaiva', true ) );
        $month_year = date( "F", $date );
        $all_events[ $month_year ][] = $query->post;    
    endwhile;
    // Sorting events by month

    foreach ( $all_events as $month_year => $events) : ?>
        <?php echo '<br><br><strong>' . $month_year . '</strong><br>';  ?>

        <?php foreach ( $events as $event ) : ?>
            <?php echo $event->post_title . '<br>';?>
        <?php endforeach;?>
    <?php endforeach;
else :
    echo ('No results.');
endif;

wp_reset_postdata(); 

Answer 1

Problem solved!

Before - Select Month

<option value="01"></option>
<option value="02"></option>
etc.

Change - Select Month

<option value="20210101"></option>
<option value="20210201"></option>
etc.

//ARGS
$selected_cat = $_GET['cat']; //category value
$start_date = $_GET['month']; //month value
$end_date = $start_date + 30;

'meta_query' => [
[
'key' => 'event_date', //acf date picker field Ymd
'value' => array($start_date, $end_date),
'compare' => 'BETWEEN',
'type' => 'DATE',
]
]