Can I delete annotations for US holidays or add annotations for my country's holidays?

At the this time, there's no way to remove the US Holidays from the list of annotations or choose a different country’s holidays.

However, if you want to add annotations for country-specific holidays, you can do so using the HTTP API. For example, if you want to create an annotation for July 18, 2016 at 5pm called “This is just a test,” the HTTP request would look like this:

https://YOUR_API_SECRET@mixpanel.com/api/2.0/annotations/create?date=2016-07-18+17%3A00%3A00&description=This+is+just+a+test

The full documentation on using the HTTP API to handle annotations.