=WEEKDAY(DATE(2024,1,7),2) is expected to return 7: January 7, 2024 was a Sunday and mode 2 numbers Monday as 1 through Sunday as 7. Omitting the second argument changes the numbering convention: =WEEKDAY(DATE(2024,1,7)) is expected 1, because Sunday is 1 by default. This distinction matters in scheduling, grouping and import validation.

Use WEEKDAY to convert an actual calendar date into a day-of-week number, consistently with a specified first-day convention. Do not confuse it with DAY, which returns the day of the month, or WEEKNUM, which returns a week number within a year. A numeric weekday does not automatically identify a holiday, working day or the cultural name of a weekday.

=WEEKDAY(date, first-day)

Apple’s WEEKDAY reference explicitly documents all three modes and the default. It does not establish every possible coercion or error code for out-of-range modes; do not assume a mode of 0, 4, or the Excel numbering schemes 11–17 are supported merely because another spreadsheet supports them.

Create one Numbers table named CalendarAudit. Make row 1 a header and fill columns A:C with the year, month and day integers below. Enter =DATE(A2,B2,C2) into D2, fill D2 down through D9, and format D as Date & Time. Dates are shown here as ISO-style labels only to eliminate regional display ambiguity. Do not type the dates in column D as text. All results in E:J are expected results independently calculated from calendar arithmetic, not observations from running Numbers.

*D is computed by Numbers; its visible formatting depends on the document locale. The J column is included for conceptual ISO comparison and should use =ISOWEEKNUM(D2) if you choose to create it; Apple’s official ISO page was inspected separately.

In E2 enter =WEEKDAY(D2,1), in F2 enter =WEEKDAY(D2,2), and in G2 enter =WEEKDAY(D2,3); fill the formulas through row 9. To reproduce the comparison columns H and I, use =WEEKNUM(D2,1) and =WEEKNUM(D2,2) respectively. Expected E2:G9 includes Monday Jan 1 as 2,1,0 and Sunday Jan 7 as 1,7,6. Python’s standard-library datetime independently established the named weekdays for the fixed dates; the Numbers formula outputs are still unexecuted expectations.

Weekend flag under a Monday-first convention: In K2 enter =IF(WEEKDAY(D2,2)>5,"Weekend","Weekday") and fill through K9. Under a Saturday/Sunday weekend rule, the expected rows 3, 5 and 9 (January 7, January 14 and October 11) say Weekend; other rows say Weekday. That is an explicitly chosen weekend policy, not a universal regional business-calendar rule.

Compact Monday-zero coding: =WEEKDAY(DATE(2024,2,29),3) is expected 3, because Thursday is index 3 after Monday = 0. This is useful for matching 0-based weekday codes in external datasets, but only when both systems share Monday as their zero day.

Conditional check with reference cells: Put a selected date in CalendarAudit::D4 (January 8, 2024), then use =WEEKDAY(D4,2)=1 in a Boolean output column. The expected result is TRUE because it is Monday. This uses a regular same-table reference; if your formula is outside CalendarAudit, choose D4 in the editor instead of hard-coding a potentially incorrect cross-table address.

Date literal hazards: Apple’s official example uses a date string like "Apr 6, 1988", but a typed string such as "04/06/1988" can be read differently under day/month versus month/day locales. The deterministic alternative is =WEEKDAY(DATE(1988,4,6),2), expected 3, without relying on parsing an English date string.

• Sunday unexpectedly displays 1 instead of 7: Your first-day argument is probably omitted or set to 1. Use 2 for Monday-first 1–7 indexing.
• Monday appears as zero: Mode 3 is designed to number Monday as zero; this is not the same as mode 2.
• An apparent date gives an error or unexpected day: Inspect its cell type and the file’s region settings; imported text may not be a real date. Try numeric DATE(...) construction to diagnose date parsing.
• Reference is from the wrong table: Verify the function editor’s highlighted input cell and the original table name before trusting a column-filled formula.
• Weekday is not a week-of-year value: Use Apple’s separate WEEKNUM or ISOWEEKNUM depending on the intended calendar rule.
• Unrecognised mode or blank input: Apple’s cited WEEKDAY page doesn’t specify an exhaustive error code/type-conversion table. Leave exact error messages to native Numbers testing rather than inventing #VALUE! behavior.
• Time of day: The official input accepts a date/time value, but the page doesn’t document tricky time-zone shifts at midnight. These fixed tests use dates at ordinary date-only boundaries.

Excel and Google Sheets also have a WEEKDAY(date, type)-style operation, but do not assume every accepted type value or default is identical: this Numbers guide validates only Apple’s documented modes 1, 2, and 3. The date value, file locale, syntax punctuation and cross-table references need application-specific testing. In Numbers on Mac, iPhone and iPad, the Apple reference documents the function generally; minimum version and detailed platform differences were not independently proven during this review.

In an English-language Numbers document, begin with = and use the comma-separated arguments shown in Apple’s Functions Help. If your device is configured to use a comma as the decimal separator, Apple’s instructions say to separate formula arguments using semicolons. Example: =WEEKDAY(D2;2) rather than =WEEKDAY(D2,2) in such a locale. Quotation marks around a date string do not remove local date-parsing ambiguity; construct examples from numeric DATE(year,month,day) inputs or genuine date-formatted cells instead.

Numbers is table-centred. When the formula is in the same table, D2 identifies that table’s cell; when referring to a different table, select the actual source cell in the formula editor so Numbers inserts the correct table-qualified reference. Do not silently assume Excel’s Sheet1!D2 syntax applies. The expected outputs below use an English Functions Help 15.4 reference path consulted 2026-10-11, not a claim that these functions first appeared in app release 15.4. Apple describes date/time functions in shared Formulas and Functions Help; this research did not establish a minimum Numbers version on macOS, iOS, iPadOS or iCloud. Native verification on each intended app/platform/version is pending.

Testing note: This guide uses documentation-derived examples, not results observed in Apple Numbers.

• Apple Numbers — function documentation
• Apple DAYNAME
• Apple WEEKNUM
• Apple Functions overview

Verification status: Official documentation checked; example outcomes were independently reasoned or arithmetically checked where applicable. Not executed in Apple Numbers.

• No genuine Apple Numbers screenshots are included; this guide does not use mock application images.