There isn’t any way to do this in ACF. What you’re going to need to do is to create a new custom widget. You can start by copying the existing calendar widget /wp-includes/widgets/class-wp-widget-calendar.php
Thanks a lot for your answer, i’m just surprised there no third party solution, because i suppose it’s a basic need, using and sorting in widget calendar with ACF date field instead of wp post_date.
The problem is really that there are no suitable hooks in the built in WP widgets that will let you do anything to modify them. I’ve run into the same thing when I’ve needed other widgets, like a date or category widget for a custom post type.