Day of the Week Calculator
Find which weekday any date falls on, with ISO week number and day-of-year.
Inputs
Enter any past or future date.
Full Date
Friday, July 4, 2025
Day of the Week
Friday
Weekday or Weekend
Weekday
ISO Week Number
27
ISO 8601 week, where week 1 contains the first Thursday of the year.
Day of Year
185
Ordinal day number within the calendar year (1 = Jan 1).
Days Left in Year
180
Leap Year
No
Next Same Weekday
2025-07-11
Step by step
Parsed date (UTC)
= 2025-07-04
Day of week index (0 = Sun)
= 5 → Friday
Day of year
= Day 185 of 365
ISO week
Week 27 of ISO year 2025
= W27
How it works
The weekday is determined by parsing the date as a UTC midnight timestamp and reading its getUTCDay() value, which avoids timezone-induced off-by-one errors. The ISO 8601 week number is computed by finding the Thursday of the week the date belongs to — ISO weeks always start on Monday and week 1 is the week that contains January 4 (equivalently, the first Thursday of the year). The day-of-year is the ordinal count from January 1.
Formulas
ISO Week 1 rule
Week 1 is the calendar week containing January 4th (or equivalently, the first Thursday of the year).
Day of year
Day of year = (date timestamp − Jan 0 timestamp) ÷ ms-per-day
Frequently Asked Questions
What is an ISO week number?
ISO 8601 weeks run Monday to Sunday and are numbered 1–53. Week 1 is the first week that contains a Thursday, which means January 1 can be in week 52 or 53 of the previous year if it falls late in the week.
Can I look up historical dates before 1970?
Yes. JavaScript's Date object handles the proleptic Gregorian calendar back to at least 100 AD for day-of-week calculations.
What does 'day of year' mean?
It's the ordinal position of the date within the calendar year. January 1 = day 1, December 31 = day 365 (or 366 in a leap year).
Why does the ISO year sometimes differ from the calendar year?
The last few days of December and the first few days of January can belong to the adjacent ISO year. For example, December 31, 2018 was in ISO week 1 of 2019.