CSS and Javascript Scripts for Widgets can be used based on widget availability. If you have created a custom widget and need to load stylesheet only if it active on front end, you will have to use is_active_widget WordPress function right after parent::__construct method has been called.

class MY_Custom_Widget extends WP_Widget {

    function __construct() {
        $widget_ops = array(
            'classname' => 'my_custom_widget',
            'description' => __( 'MY Custom Widget' )
            __( 'My Custom Widget' ), 
    // rest of the widget codes

Now for using Scripts for Widget outside of the Widget class, ex: loading stylesheet for calendar widget, you just need to figure out the widget unique id.

function w4dev_load_calendar_scripts() {
    if ( is_active_widget( false, false, 'calendar', true ) ) {
        wp_enqueue_script( 'jquery' );
        wp_enqueue_style( 'something', 'stylesheet_url' ); 
add_action( 'wp_head', 'w4dev_load_calendar_scripts');