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In our computed calendars, we simply count days from our arbitrary and fictitious starting point of Monday 1 January 1 A.D. (Gregorian), which equates to the real 3 January 1 A.D. (Julian), where "A.D." is acronym for Anno Domini, in the US sometimes quoted "c.e." for Common Era. Thus, we display sections of the current civil (Gregorian) calendar.
Because the Julian Calendar had been used until the Gregorian Calendar was instituted on 15.10.1582 A.D. (Gregorian), prior Gregorian dates are fictitious, albeit displayed by our calendar. They should be converted to Julian dates, which we do not do for two practical reasons; early on, there was no seven-day week; and, the actual transition from the Julian to Gregorian calendar having varied among countries. In fact, it was only the Catholic states of Europe that converted on the original date. The protestant countries of continental Europe resisted until 1700 and 1701; Great Britain and its colonies - including the American colony - waited until 1752 to adopt the new style, i.e. Civilian Calendar. Some other countries waited much longer, yet.
In the western world, the week of seven days had not existed until it was instituted by the Council of Nicaea, in 325 A.D.
No holidays before the historical year of 1783 are shown in our calendar for the USA. No Roman Catholic holidays are shown before 1582 and no protestant holidays before 1701.
Many, in some European countries most, legal holidays are calendrically pivoted at Easter, whose definition anchors many holidays of western countries. The algorithm we use for Easter is a wholly correct, arithmetical, procedure that was first published in the British science journal Nature (1876). Before that, rules for Easter had been considered by various mathematicians, including the great C.Friedrich Gauss (1777 - 1855).
Easter was originally defined by the Council of Nicaea as first Sunday after the first fullmoon on or after the spring (i.e. vernal) equinox, at the meridian of Venice. Such being an insufficient definition, it resulted in paradoxes and became another cause for revising the Julian calendar. Both, Easter and the calendar, were then redefined by a commission appointed by Pope Gregorus XIII. A self consistent system was then developed whose astronomical basis is derived in the arithmetical tables contributed by Gregorus' principal advisor, the German Jesuit astronomer Clavius. He seems to have overlooked virtually nothing and Easter Sunday occurs between 22 March and 25 April, inclusive, for thousands of years, as it had before, in the Julian Calendar. A moon crater is named after Clavius.
Years past 9998 will be rejected when attempted as data entry.