Holiday calendars are so country-specific that the useful advice is about how to plan around them rather than listing dates. Every country has a handful of major holidays when life genuinely stops: national days, the main religious festivals, and New Year periods, which in some countries follow lunar calendars and shift dates each year.
During the big ones, expect government offices, banks, and many businesses to close, sometimes for several days in a row. Visa appointments, embassy services, and official processing pause too, which catches out a lot of expats with pending paperwork. Transport runs the other direction: it does not stop, it overloads, since everyone travels home to family. Book trains, flights, and buses well ahead if you must move during a major holiday, and expect premium prices.
Before planning anything important, search the official public holiday list for your specific country and year, which governments publish online, and note that regional holidays can close things locally even when the country as a whole is open. Also remember that a holiday falling near a weekend often becomes a long bridge weekend with extra closures.