Example snippet which can be used to change the 'View Details' button text based on the event status.

tw_ee_view_details_button.php

<?php //Please do not include the opening PHP tag if you already have one.

function tw_ee_view_details_button($btn_text, $event) {
    // Check if the event is expired.
    if ($event->is_expired()) {
        return 'View Details';
    } 
    // Check if the event is sould out.
    if ($event->is_sold_out()) {
        return 'Sold Out';
    }
    // Check if the event has tickets available for sale.
    // This check includes both upcoming AND expired ticket sale dates.
    // So if the event has tickets all set with a 'registration start' date that is
    // in the future or all tickets have expired (but the event is not), the button shows 'Registration Closed'.
    if (! $event->tickets_on_sale() ) {
        return 'Registration Closed';
    }
    // Return the 'default' string you want to use when the above conditions aren't true.
    return 'Register now';
}
add_filter ('FHEE__EE_Ticket_Selector__display_view_details_btn__btn_text', 'tw_ee_view_details_button', 10, 2);