Transform

Extract day of week names or numbers (Monday, Mon, 1–7) from date columns

Derive day-of-week names or numeric values from date columns for cohort analytics.

🛡️ Your rows never leave your browser.
Why Onfyt: Supports configurable week start conventions (ISO 1=Monday vs US 1=Sunday) for seamless integration with accounting calendars.

Interactive Sample Data

sale_idsale_daterevenue
12026-10-051200
22026-10-06850
32026-10-103100

How this tool works

Retail sales analysis, foot traffic studies, and employee scheduling models frequently need to group transactions by the day of the week. Knowing whether sales peak on Fridays or Sundays allows marketing teams to optimize ad spend and warehouse managers to staff accordingly. However, raw transaction tables rarely store the weekday; they record calendar dates like '2026-10-05'. Calculating weekdays in spreadsheets using =TEXT(A2, "dddd") works on modern desktop Excel but breaks when files are imported into automated databases, SQL loaders, or BI engines like PowerBI or Looker that require explicit tabular attributes. DataFix parses calendar dates in any standard format (ISO 8601, YYYY-MM-DD, MM/DD/YYYY, DD/MM/YYYY) and appends a dedicated weekday column. You can choose your output format: full English name (Monday, Tuesday), three-letter abbreviation (Mon, Tue), ISO weekday number (1 for Monday through 7 for Sunday), or North American weekday number (1 for Sunday through 7 for Saturday). Extracting day-of-week names and numeric indices from date timestamps enables day-parting analysis, retail traffic modeling, and scheduling optimization.

Things that usually go wrong

  • Dates in ambiguous formats (01/02/2024 — is that Jan 2 or Feb 1?) should be standardised first.
  • The week start day (Sunday or Monday) affects the numeric output.

Edge cases we tested

🧪 Leap year leap day dates ('2028-02-29')

Correctly evaluated as Tuesday without date math errors.

🧪 Timestamps with time components ('2026-10-05 23:45:00')

Evaluates the calendar day without tripping on time strings.

🧪 Invalid date strings ('2026-02-30')

Flagged cleanly as invalid date rather than returning erroneous weekdays.

Frequently Asked Questions

What is the numeric mapping?

0 = Sunday, 1 = Monday, …, 6 = Saturday. If you prefer Monday = 1 (ISO), select 'ISO weekday'.

Can I filter rows by weekday?

After adding the weekday column, use Filter CSV Rows to keep only, say, rows where weekday = 'Monday'.

Can I use the weekday column to filter for only weekdays (Monday–Friday)?

Yes — after adding the weekday column, use Filter CSV Rows with condition: weekday != 'Saturday' AND weekday != 'Sunday'.

Is the first day of the week Sunday or Monday?

The numeric output follows ISO 8601 (Monday = 1 … Sunday = 7) when 'ISO mode' is selected, or US convention (Sunday = 0 … Saturday = 6) in default mode.

Related tools

Transform

Column to Comma-Separated List

Extract a CSV column and convert it to a comma-separated list for SQL IN clauses or code arrays.

Open Tool →
Transform

Comma-Separated List to Column

Convert a comma-separated, line-separated, or SQL IN list into a single-column CSV.

Open Tool →
Transform

Lines to CSV

Convert plain-text newline-delimited records into structured CSV tables.

Open Tool →