Polars · Module

.dt.weekday

Extracts the ISO weekday number from Date or Datetime expressions. Monday is 1 and Sunday is 7, and the result has Int8 data type.

How it works

Call dt.weekday() on a date- or datetime-valued expression to produce the ISO weekday number for each value. The result is an Int8 expression, with Monday represented by 1 and Sunday by 7; it can be added to a DataFrame using with_columns.

Syntax

Expr.dt.weekday() → Expr

Returns

Expr of data type Int8 containing the ISO weekday number for each Date or Datetime value.

Key points

The method derives the weekday from the underlying date representation.
It supports both Date and Datetime columns.
The numbering is ISO-based: Monday maps to 1 and Sunday maps to 7.
The returned expression has the Int8 data type.
The method takes no arguments and returns an Expr.

Common mistakes

The expression must operate on a Date or Datetime column; other logical data types are outside the documented applicability.

Example: Extract ISO weekdays

import polars as pl
from datetime import date

df = pl.DataFrame({"date": [date(2024, 1, 1), date(2024, 1, 7)]})
result = df.with_columns(
    pl.col("date").dt.weekday().alias("weekday")
)
print(result)

Adds an ISO weekday-number column: Monday is 1 and Sunday is 7.

When to use it

Calendar-based aggregation

Add a numeric weekday column for grouping or reporting.

Weekend and weekday filtering

Filter records to weekdays or weekends.

Temporal feature engineering

Build weekday features for analytics or machine-learning pipelines.

Interview questions

QWhat does Expr.dt.weekday() return?

Expr.dt.weekday() extracts the ISO weekday from a Date or Datetime expression. It returns an Expr whose data type is Int8, using 1 for Monday through 7 for Sunday.

Follow-up: How would you retain the original date while adding the weekday? Use with_columns(pl.col("date").dt.weekday().alias("weekday")).

QWhat numbering convention does it use?

The output follows ISO weekday numbering: Monday is 1 and Sunday is 7. This makes weekday comparisons independent of locale-specific Sunday-first numbering.

Follow-up: How would you select weekend rows? Filter where the derived weekday is greater than 5, such as pl.col("date").dt.weekday() > 5.

QHow is this method typically used in a DataFrame transformation?

It is an expression method, so it is evaluated as part of a Polars expression pipeline rather than immediately extracting one scalar. It can therefore be used in transformations such as with_columns and filter.

Follow-up: What output type should downstream code expect? An Int8 expression, not a string containing a weekday name.

Expr.dt.day
Expr.dt.week
Expr.dt.ordinal_day
Series.dt.weekday

Source: polars.Expr.dt.weekday — Polars documentation. Checked 2026-10-15.