Thanks for reaching out and welcome to the forums, based on your questions you can change this setting via the WordPress default settings you need to go to:
Settings > General
Or
/wp-admin/options-general.php
Then go to the bottom and you should be able to locate the section where you can change the default start date of the week and this will update how the calendar starts as well.
After that you only need to Save the Changes and you should be all set.
Please let me know if you have any other questions about this issue or if there’s anything else I can do for you.
Thanks,
Crisoforo