day()

Sets the day of the week. Returns a new instance (immutable).

Syntax

.day(value: number): FduInstance

Parameters

value: number - Day of week (0-6, where 0 = Sunday, 6 = Saturday)

Returns

FduInstance - New instance with the updated day of week

Examples

Setting Day of Week

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

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

date.day(0); // Previous/next Sunday: 2025-10-05
date.day(1); // Stays Monday: 2025-10-06
date.day(5); // Next Friday: 2025-10-10
date.day(6); // Next Saturday: 2025-10-11

Setting to Weekend

const monday = fdu("2025-10-06");

const saturday = monday.day(6); // 2025-10-11
const sunday = monday.day(0); // 2025-10-05

Preserving Time

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

const friday = date.day(5);
friday.format("YYYY-MM-DD HH:mm:ss"); // '2025-10-10 14:30:45'

Day Values

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

Behavior

The method calculates the difference between the current day and target day, then adjusts the date accordingly:

const wednesday = fdu("2025-10-08"); // Wednesday (day 3)

wednesday.day(0); // 2025-10-05 (Sunday, -3 days)
wednesday.day(6); // 2025-10-11 (Saturday, +3 days)
wednesday.day(3); // 2025-10-08 (same day, no change)

Immutability

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

const original = fdu("2025-10-06"); // Monday
const modified = original.day(5); // Friday

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

Common Patterns

Find next occurrence of a day

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

const monday = fdu("2025-10-06");
const nextFriday = nextDay(monday, 5); // 2025-10-10

Schedule weekly meeting

const today = fdu();
const nextMeeting = today.day(3).add(9, "hour"); // Next Wednesday at 9 AM

console.log(`Meeting: ${nextMeeting.format("YYYY-MM-DD HH:mm")}`);

Get start/end of work week

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

const workWeekStart = date.day(1); // Monday
const workWeekEnd = date.day(5); // Friday

console.log(
  `Week: ${workWeekStart.format("MMM D")} - ${workWeekEnd.format("MMM D")}`,
);

Find last occurrence of a day

function lastDay(date: FduInstance, targetDay: number): FduInstance {
  const result = date.day(targetDay);
  return result.isAfter(date) ? result.subtract(7, "day") : result;
}

const friday = fdu("2025-10-10");
const lastMonday = lastDay(friday, 1); // 2025-10-06