In this article -

As the calendar invitation mails are sent by the calendar app, e.g Google Calendar, we have control only over the event name (summary) and its description. If you do not dabble with code, we recommend using the services of a PHP developer who has good knowledge in WordPress to do the customisations. The developer should copy the original desired templates from the plugin and place it in child theme folder and modify it based on your needs.

All calendar event content templates can be found in the [WP Site root]/wp-content/plugins/wpcal/templates/calendar_events folder.

common_calendar_event_plain.php

Make sure the 'Invitee notification type' option of the event type is set to 'Calendar invitation' and that the 'Add booking to...' calendar (under WPCal.io › Settings › Calendars) is added.

In this article we gonna customise common_calendar_event_plain.php template

Locate the plugin's calendar event content template in wpcal plugin's templates folder.

[WP Site root]/wp-content/plugins/wpcal/templates/

in this case the template we going to customise is calendar_events/common_calendar_event_plain.php

Full path to the desired template file is

[WP Site root]/wp-content/plugins/wpcal/templates/calendar_events/common_calendar_event_plain.php

It is highly recommended to place the overriding (customised template) file inside a child theme so that it will not get removed during theme updates.

In our case, the child theme is twentytwentyone-child

Place the copied file in the child theme by creating a 'wpcal' folder as mentioned below

[WP Site root]/wp-content/themes/twentytwentyone-child/wpcal/calendar_events/common_calendar_event_plain.php

To verify whether you have successfully overridden the template file, go to WP Admin › WPCal.io › Settings › General

Once your customised template displays in the list, you are all set to do the customization.

Now you can customise the file [WP Site root]/wp-content/themes/twentytwentyone-child/wpcal/calendar_events/common_calendar_event_plain.php and make sure to set appropriate values for $summary and $descr_txt before the following line which can be located near the end of file.

$template_result = ['summary' => $summary, 'descr' => $descr_txt];

We strongly recommend you to do the email customisations in the staging site and once all changes are done and tested, deploy it to the production site.