What regional holidays should an employee follow?

An employee should always follow the regional holidays based on their work location and not that of the client. 

See also: Where can I find information about National and Regional Holidays?

If your company allows employees to observe holidays from a different country, these should be granted in addition to the holidays for your location. You will need to submit a time off request for these days using the Other leave type.

For Customers, keep in mind that an employee's country laws may entitle them to overtime pay if they work on a national holiday.