Make all events default to same day

you need custom php coding for this, you can modify template file at templates/forms/event-editor.php

to use templates: https://wp-events-plugin.com/documentation/using-template-files/

Coding this is beyond me unfortunately. Can anyone help with the code that would do this?

unfortunately, I don’t have a working code or other similar snippet for this but basically you need to modify that template and to be specific templates:

for normal event
– wp-content/plugins/events-manager/templates/forms/event/when.php

for recurring event
– wp-content/plugins/events-manager/templates/forms/event/when-with-recurring.php

and then add new javascript to auto check checkbox of all day

to use templates: https://wp-events-plugin.com/documentation/using-template-files/