WooCommerce Custom fields: Customizing jQuery Datepicker Plugin

I am using WooCommerce with a plugin that provides custom fields on a product page. One of the options is a date picker. Although, the product sold is only available on specific days of the week (e.g. Monday and Wednesday).

I understand how to get this effect, and an extensive google search gave me heaps of options. But I really don't understand where to put it, and how to override the plugin files.

How do I edit the plugin's jQuery files to have specific days selectable in the date picker?
Where would I find the files to achieve this effect?

I have been searching forever and decided to ask the community

Any help is highly appreciated.
Thanks.

via Chebli Mohamed