Diverse Holiday Observance
I worked in a very multicultural company in the Middle East for 10 years. While I leave it to any country to decide which particular holidays are "National", I do think that the fundamental under-riding issue is "days off" for observance.
In that regard companies and HRM can do quite a bit! One of the easiest solutions is to separate ethnic and religious "holidays" from the corporate "days off" schedule. Allocating an equal number of "Holidays" apart from vacation time for all employees allows them to take time off when THEY benefit most (subject to staffing requirements, of course). "Coverage" by other employees becomes easier to manage as well. Only cases where the majority of employees may want the same days off may require extra consideration - including just making it a corporate day off.
Some people prefer to "work" the slower-paced holidays instead of engaging in "holiday" activity. Best regards, Steven.