If you have ever scheduled something across two countries and had a public holiday quietly wreck the plan, you have run into one of three sources of variation: fixed-but-different dates, calculated holidays that move every year, and regional holidays that only apply in part of a country.
Fixed dates, different countries
Plenty of holidays are simply different by design -- national independence days, founding days, and similarly fixed civic holidays land on a specific calendar date that has nothing to do with any other country's calendar. These are the easy case: same date every year, just look it up once.
Calculated holidays that move every year
Many widely observed holidays are defined by a rule rather than a fixed date -- 'the fourth Thursday of November,' or holidays tied to lunar or religious calendars that do not align with the Gregorian calendar year. That means the calendar date shifts every year, sometimes by weeks, which is exactly the kind of thing that is easy to get wrong from memory or an old calendar printout.
Regional and substitute holidays
Some countries observe different official holidays by state, province, or region, so a date that is a public holiday in one part of a country may be a normal business day in another. Several countries also apply a 'substitute holiday' rule: if a holiday falls on a weekend, the next available weekday becomes the observed holiday instead -- which means the observed date and the traditional date are not always the same.
Checking the current year
Because of all three of these, the only reliable way to confirm a holiday date is to check the current year for the specific country you care about, rather than assuming it repeats from last year. The public holiday checker looks this up live by year and country code, pulling from a public holiday data source rather than a static list, so it reflects calculated and substitute-holiday dates correctly for the year you actually need.