Query between dates using Date Picker fields

The Codex lists all of the comparison options on the WP_Query page.

If you want events with a start date BETWEEN a range of dates, OR events with an end date BETWEEN a range of dates, something like this should work –

'relation' => 'OR',
    array(
        'key' => 'event_start_date',
        'value' => array( $beginning_of_range, $end_of_range ),
        'compare' => 'BETWEEN',
        'type' => 'DATE'
    ),
    array(
        'key' => 'event_end_date',
        'value' => array( $beginning_of_range, $end_of_range ),
        'compare' => 'BETWEEN',
        'type' => 'DATE'
    )