Date input format problem
Rather than using Feeds, I suggest using the core Migrate module.  There is also a core plugin for converting dates during migration, FormatDate.  This will allow you do specific the input format and the output format.  I believe, this process configuration would work in your case:

process:
  field_date:
    plugin: format_date
    from_format: 'F j, Y'
    to_format: 'Y-m-d'
    source: event_date


If Feeds is your preferred method, then you can try the Feeds Tamper module.  Based on the plugin list for Drupal 8, I think you would need to do a "String to Unix timestamp" tamper on the field, followed by a "Unix timestamp to Date" tamper, using 'Y-m-d' as the format string. However, I haven't used Feeds and Feeds Tamper in a long time, and would typically write custom tampers for something like this.