I have not come across the expression 'a highly international holiday'. When I read the question 'Why do we say "a highly international holiday ...?', my first reaction was to respond 'We don't'.
Judaism has "High Holidays", specifically Yom Kippur and Rosh Hashanah. However, I would not describe them as "high international holidays". I would consider "international High Holidays" since Judaism crosses all borders, but that would not be common. I agree with 5jj's thoughts about "highly international holiday".
The original sentences is: (Written in Voxy Page on Facebook)
Mardi Gras is a highly international holiday. Some of the other major celebrations take place in Germany (as "Karneval"), Italy (as "Martedi Grasso"), Trinidad (as "J'Ouvert"), and Mexico (as "Martes de Carnaval")."
In my opinion, that's a poor use of the word "highly". I would have written something like "Mardi Gras is a holiday/festival which is celebrated internationally" or "Mardi Gras is an internationally-celebrated holiday/festival".
I don't think I'd use 'highly intenational' at all, but I don't object to it in the way it's used in these sentences. It would even just about work if I read "Mardi Gras is a highly international festival in Rio. Tens of thousands of vistors from all over the world flock to the city for the event" However, in the sense of it being celebrated all over the world, I'd go for ems's suggestions or say that it is 'a truly international holiday'.
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