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Date & Timedates

Day of the Week Finder

The day of the week for any date, with the ISO week number. ISO weeks start on Monday and week one contains the first Thursday, so early January can belong to the previous year.

Also called: what day was i born, weekday calculator.

Day of the week
Tuesday

2026-08-18 is a Tuesday. It is day 230 of the year, in ISO week 34, and 229 days after the reference date.

Day of the year
230
ISO week number
34
Day number, Sunday is 0
2
Quarter
Q3
Relative to 2026-01-01
229 days after
How this is calculated
Method and background

How this is calculated

The day of the week comes straight from the calendar. The ISO week number is the part with a rule worth knowing: weeks run Monday to Sunday, and week one is the week containing the first Thursday of the year. That means 1 January can fall in week 52 or 53 of the preceding year, which catches out anyone doing weekly reporting across a year boundary.

the ISO rule, which is why 1 January can fall in week 52 of the previous year
d
The date

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a Tuesday in August

Date
2026-08-18

Day of the weekTuesday

Verified against the calendar

Open this example

Method and limits

What it assumes

  • The proleptic Gregorian calendar, at UTC.

What it deliberately does not model

  • Dates before the Gregorian reform in 1582 are extrapolated rather than historical.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why is 1 January sometimes in week 52?
Because ISO week one is the week containing the first Thursday. If 1 January is a Friday, Saturday or Sunday, that week belongs to the previous year.