Is Holidays a dimension?

On the one hand, I can put US_Holiday_Flag, CA_Holiday_Flag, MX_Holiday_Flag, etc. into the Date dimension. On the other hand, I may not know 10 years into the future what all the holidays might be in all the countries I need to support. I've seen ready-made table scripts with the former. Is there standard school on this? Or, at least, a healthy debate?

Yes it is, but it can be a outrigger dim to the date dim since you want to support multiple holidays.

This can be a real "banana skin" area. Watch out for countries like Australia where each state has it's own public holiday dates and NZ with County holidays.

The other tricky aspect to consider in your design is "who's holiday" e.g. you have a customer in location X serviced by a team in location Y - which public holidays should apply?

Good luck!