EE3 modify widget dates

This line controls the individual output for each event:

<li><a href="<?php echo $registration_url; ?>"><?php echo stripslashes_deep($event->event_name) ?> - <span class="widget-event-date"><?php echo event_date_display($event->start_date) ?></span></a>

By simply changing the event_date_display($event->start_date) to something like date('d-m-Y', $event->start_date) you can modify the actual format output.

So the following could work for you:

<li><a href="<?php echo $registration_url; ?>"><?php echo stripslashes_deep($event->event_name) ?> - <span class="widget-event-date"><?php echo date( 'F jS', strtotime($event->start_date) ) ?> - <?php echo date( 'jS Y', strtotime($event->end_date) ) ?></span></a></li>