Pick a year and country: the holidays are computed from the actual rules, so any year works, past or future.
Most holidays are rules, not fixed dates: 'the fourth Thursday of November', 'the Monday on or before May 24', 'the first Monday of October', plus the observed-date shifts when a fixed holiday lands on a weekend. This lookup computes them from the legislated rules, including the Easter computus for Good Friday and Easter Monday, which is why it works for any year rather than a printed table's range. The US list is the eleven federal holidays; the UK list is the England & Wales bank holidays (Scotland and Northern Ireland differ on a few); Canada is the federal list (provinces add their own); Australia is the national public holidays (states add more).
Two honest caveats: 'observed' dates, when government offices close for a weekend holiday, are shown for US federal holidays because employers commonly follow them, but private employers are not bound by the federal calendar. And legislatures occasionally change holiday law; the rules encoded are the long-standing current ones.
Why does a holiday show two dates some years? That is the observed rule: when a fixed-date US federal holiday falls on Saturday it is observed the Friday before; on Sunday, the Monday after. The actual date is the holiday; the observed date is when federal offices close.
Are state holidays included? Not in these national lists, US states, Canadian provinces and Australian states each add holidays (and a few move them). The national core computed here is the shared baseline.
How is Easter calculated? By the standard Western computus, the algorithm churches have used for centuries to place Easter Sunday, from which Good Friday (two days earlier) and Easter Monday (one day later) follow.