Kalkulator dnia tygodnia
The detailed guide below is currently available in English.
How the weekday is found
Every date in the Gregorian cycle lands on a fixed weekday that repeats every 400 years. The calculator anchors on that cycle, so any date from year 1 to 9999 resolves instantly — useful for checking birthdays, historical events, or which weekday a future anniversary falls on.
ISO week numbers
The ISO week number follows ISO-8601: weeks run Monday through Sunday, and week 1 is the week containing the year's first Thursday. Early January dates can therefore belong to the final week of the previous year, and December 29–31 can already be week 1 of the next year — that is correct ISO behavior, not an off-by-one error.
Częste pytania
Why does January 1 sometimes show week 52 or 53?
Under ISO-8601, a January 1 that falls on a Friday, Saturday, or Sunday belongs to the last week of the previous year, because week 1 must contain the first Thursday of the year.
Which calendar does the calculator use?
The proleptic Gregorian calendar — the same rules extended backward before 1582. Dates from the Julian era may not match historical records for that reason.