Weekday of New Year's Day

The result is derived entirely from the supplied date under Gregorian calendar rules, so it does not depend on the current date, a server clock, a browser timezone, or an external calendar service. Along with the weekday name, the response includes its ISO weekday number and the exact January 1 date used in the calculation.

Use any date to identify the relevant year

Enter one explicit calendar date in the ISO form YYYY-MM-DD. The calculator validates the complete date, including the number of days in its month and the special February rule for leap years, then takes the year portion as the year to evaluate. For example, an input in August 2026 asks a different question from the weekday of that August date: the calculation deliberately returns the weekday for January 1, 2026. This makes the tool convenient when a record, deadline, invoice, or event already contains a date and you need information about the beginning of that same year. Requiring a complete, valid date also prevents ambiguous inputs such as 03/04/26, whose month and day depend on regional convention. Years are accepted from 0001 through 9999, using the proleptic Gregorian calendar consistently across that range. The output repeats the accepted input, identifies the extracted year, and provides the normalized New Year's Day date so the basis of the answer remains visible rather than implicit.

Understand the deterministic weekday calculation

The calculation counts the days in all complete years before the supplied year. Ordinary years contribute 365 days, while leap years contribute one additional day according to the Gregorian rule: a year divisible by four is a leap year unless it is divisible by one hundred, except that years divisible by four hundred remain leap years. The total is reduced modulo seven, using Monday as ISO weekday 1 and Sunday as ISO weekday 7. That arithmetic directly identifies the weekday on which January 1 begins. No system clock, locale setting, timezone database, random value, network request, or JavaScript Date object participates in the result. Consequently, the same valid input produces the same JSON response in an API worker, a browser in another country, or a test process configured with a different timezone. The response includes both a readable English weekday name and the ISO weekday number, making it useful for display while retaining a compact numeric value for sorting, branching, or integration with systems that already follow ISO weekday conventions.

Apply the result in planning and automation

Knowing the weekday of January 1 helps explain how a calendar year aligns with recurring weekly schedules. It can support annual planner generation, reporting-period setup, archival checks, rota templates, and simple calendar trivia. In an automated workflow, send the date already associated with a record and use the returned ISO weekday number to choose a layout or schedule branch. A result of 6 or 7 indicates that New Year's Day is on a weekend under the common Monday-to-Friday working pattern, while values 1 through 5 place it on a weekday; the capability reports calendar facts and does not assume which days your organization treats as working days or holidays. Historical users should also note that the calculation applies Gregorian rules backward for consistency, even for places and years that historically used another civil calendar. The browser execution is available without charge, while an API request costs $0.002. Because the output is determined from the input alone, it is also suitable for reproducible builds, cached reports, regression fixtures, and auditable data transformations.