weekday()

Sets the locale-aware day of the week based on the current locale's week start. Returns a new instance (immutable).

Syntax

.weekday(value: number): FduInstance

Parameters

value: number - Locale-aware day of week (0-6, where 0 = first day of week in locale)

Returns

FduInstance - New instance with the updated weekday

How It Works

Unlike day() which always uses Sunday as day 0, weekday() respects the locale's weekStart setting:

English (weekStart: 0): 0 = Sunday, 1 = Monday, ..., 6 = Saturday
Custom locale (weekStart: 1): 0 = Monday, 1 = Tuesday, ..., 6 = Sunday

Examples

English Locale (Sunday start)

import { fdu } from "@pyyupsk/fdu";

const date = fdu("2025-10-06").locale("en"); // Monday

date.weekday(0); // Sunday (2025-10-05)
date.weekday(1); // Monday (2025-10-06)
date.weekday(5); // Friday (2025-10-10)
date.weekday(6); // Saturday (2025-10-11)

Monday-start Locale

import { fdu, registerLocale } from "@pyyupsk/fdu";

// Register custom locale with Monday as first day
registerLocale("custom", {
  name: "custom",
  weekStart: 1, // Monday
  // ... other locale properties
});

const date = fdu("2025-10-05").locale("custom"); // Sunday

date.weekday(0); // Monday (2025-10-06)
date.weekday(4); // Friday (2025-10-10)
date.weekday(6); // Sunday (2025-10-05)

Comparison: day() vs weekday()

const date = fdu("2025-10-06").locale("en"); // Monday

// Using day() - always Sunday-based
date.day(0); // Sunday: 2025-10-05
date.day(1); // Monday: 2025-10-06

// Using weekday() - locale-aware (English = Sunday start)
date.weekday(0); // Sunday (first day): 2025-10-05
date.weekday(1); // Monday (second day): 2025-10-06

With Monday-start locale:

const date = fdu("2025-10-06").locale("custom"); // Monday

// Using day() - still Sunday-based
date.day(0); // Sunday: 2025-10-05
date.day(1); // Monday: 2025-10-06

// Using weekday() - respects Monday start
date.weekday(0); // Monday (first day): 2025-10-06
date.weekday(6); // Sunday (last day): 2025-10-05

Preserving Time

const date = fdu("2025-10-06T14:30:45").locale("en"); // Monday 2:30 PM

const firstDayOfWeek = date.weekday(0);
firstDayOfWeek.format("YYYY-MM-DD HH:mm:ss"); // '2025-10-05 14:30:45'

Weekday Values (Locale-dependent)

English Locale (weekStart: 0)

Value	Day	Note
0	Sunday	First day
1	Monday
2	Tuesday
3	Wednesday
4	Thursday
5	Friday
6	Saturday	Last day

Monday-start Locale (weekStart: 1)

Value	Day	Note
0	Monday	First day
1	Tuesday
2	Wednesday
3	Thursday
4	Friday
5	Saturday
6	Sunday	Last day

Immutability

.weekday() returns a new instance and never modifies the original:

const original = fdu("2025-10-06").locale("en"); // Monday
const modified = original.weekday(0); // Sunday

original.format("dddd"); // 'Monday' (unchanged)
modified.format("dddd"); // 'Sunday'

Common Patterns

Get first/last day of locale week

const date = fdu("2025-10-08").locale("en"); // Wednesday

const firstDay = date.weekday(0); // Sunday (2025-10-05)
const lastDay = date.weekday(6); // Saturday (2025-10-11)

console.log(`Week: ${firstDay.format("MMM D")} - ${lastDay.format("MMM D")}`);

Locale-aware work week

// For English locale (Sunday start), work week is days 1-5
const date = fdu().locale("en");
const workWeekStart = date.weekday(1); // Monday
const workWeekEnd = date.weekday(5); // Friday

// For Monday-start locale, work week is days 0-4
const dateCustom = fdu().locale("custom");
const workStart = dateCustom.weekday(0); // Monday
const workEnd = dateCustom.weekday(4); // Friday

International scheduling

function scheduleWeeklyMeeting(
  locale: string,
  meetingDay: number,
): FduInstance {
  return fdu().locale(locale).weekday(meetingDay);
}

// US team: Wednesday (day 3 in Sunday-start week)
const usWeekly = scheduleWeeklyMeeting("en", 3);

// EU team: Wednesday (day 2 in Monday-start week)
const euWeekly = scheduleWeeklyMeeting("custom", 2);

Find next locale-specific weekday

function nextWeekday(date: FduInstance, targetDay: number): FduInstance {
  const result = date.weekday(targetDay);
  return result.isBefore(date) || result.isSame(date, "day")
    ? result.add(7, "day")
    : result;
}

const date = fdu("2025-10-06").locale("en"); // Monday
const nextFirstDay = nextWeekday(date, 0); // Next Sunday