Reverse related entry with Solspace Calendar
after reading and re-reading the docs got this working:
{% set relatedDates = craft.calendar.events.relatedTo(entry).all() %}

{% for relatedEventEntry in relatedDates %}
 {{ relatedEventEntry.title }} - {{ relatedEventEntry.startDate.format(&quot;l, F j, Y \\a\\t g:ia&quot;) }}
      ({{ relatedEventEntry.duration.humanReadable }})
      in {{ relatedEventEntry.calendar.name }} calendar.
      {% if relatedEventEntry.repeating %}
        Repeats {{ relatedEventEntry.readableRepeatRule }}
      {% endif %}
{% endfor %}  

this is called from the projects/_entry template