add_action( 'init', 'my_function' );
function my_function() {
if ( ! wp_next_scheduled( 'your-action-hook' ) ) {
wp_schedule_event( strtotime( '00:00:00' ), 'daily', 'your-action-hook' );
}
}
add_action( 'your-action-hook', 'your_callback_function' );