Perpetual Calendar (1583+)

The page lays out the twelve months of any year from 1583 to 9999, with Brazilian national holidays marked and the ISO week number on each day's label. The lower bound is not arbitrary: October 1582 is the month in which the Gregorian reform dropped ten days in the countries that adopted it immediately, and a calendar ignoring that would show days that never existed.

The movable holidays all derive from Easter, computed with the Meeus, Jones and Butcher algorithm. From it, Carnival Tuesday falls 47 days earlier, Good Friday 2 days earlier and Corpus Christi 60 days later. Worth the caveat that Carnival is not a national holiday in law but a discretionary day off — the page marks it because in practice the country stops, but anyone drawing up a work roster needs to know the difference.

The ISO week on the label follows the standard's rule, which is not the naive count of dividing by seven: week 1 is the one containing the year's first Thursday, which sometimes puts early January in week 52 or 53 of the previous year. A curiosity the tool reveals: there are only fourteen possible year layouts — seven starting weekdays times leap or common — so 2026 matches some year you have already lived through.