Day of the Week Calculator (What Day Was It?)
Enter any date to find the day of the week it falls on, plus the day of the year, ISO weekday number, and the same date across other years.
- Free, no account
- No watermark
- No usage limit
About the Day of the Week Calculator
Enter any date and this Day of the Week Calculator tells you the weekday it lands on, free and instant, worked out entirely in your browser. Ask it about July 4, 1990 and it comes back with Wednesday. That part is easy, honestly your phone does it too.
The reason people reach for a dedicated tool is the stuff a bare weekday answer skips. Which Thursday of the month is it, the 2nd or the 3rd, because that's what the meeting rule actually says. Will the Saturday you picked for next year still be a Saturday. How does the day slide as you walk your birthday back through the decades. This one nails all of those right next to the plain weekday, for any date from year 1 to 9999, so a birthday from the 1930s is no harder than one from last week. Nothing you type ever leaves your machine, and once the page loads it keeps working offline.
How to use
- Enter a date. Use the picker, or tap Use today to drop in the current date.
- Read the big weekday. It shows in large text with the full spelled-out date underneath.
- Check the extras. Weekday numbers (Sunday-based and ISO), the day of the year, days left in the year, the ISO week, and which occurrence of that weekday it is in the month.
- Scan the years table. The same month and day across the years around yours, each with its weekday, so you can watch the day drift.
- Copy it. Grab the plain-text result and paste it wherever you need it.
Change the date and everything recalculates on the spot, so you can jump from your birthday to your kid's without reloading a thing.
What a plain weekday answer leaves out
Say you run a meeting on the 2nd Tuesday of every month, or a contract clause reads "the third Friday." A weekday name on its own doesn't get you there, you still have to sit and count. The tool does the counting for you, it tells you a date is the 2nd Tuesday, or the 4th Saturday, of its month, which is the readout I actually built it around. That's the piece a paper calendar makes you tick off by hand, and it's exactly what recurring-schedule rules turn on.
The years table is the other half of it. It lays the same month and day out across a band of years on either side, weekday for each, so the drift is right there in front of you instead of something you have to imagine. The drift comes from the leftover days. A common year is 365 days, and 365 leaves 1 when you divide by 7, so an ordinary year nudges every date forward one weekday. A leap year has 366, remainder 2, so it jumps two. Your birthday marches forward a day most years and skips one whenever a February 29 sits in between. Handy if you're eyeing a weekend date a year or two out, check the candidate actually lands on the weekend before you book the venue.
February 29 gets careful handling because it's the one date that isn't there every year. Enter it and the years table won't pretend otherwise, the real leap years show a weekday and the common years are flagged as having no such date. Same deal for a date that never existed at all, a February 30 or an April 31, the tool tells you plainly it isn't a real calendar date instead of coughing up a guess.
Two weekday numbers, and the off-by-one they cause
There isn't one agreed way to number the days, which turns into a real trap the moment you do anything programmatic with them, which is why both counts appear here. The Sunday-based count runs 0 to 6 with Sunday as 0, and it's what most US spreadsheets and a lot of programming languages hand back by default, JavaScript's getDay() being the one people trip over most. The ISO 8601 count runs 1 to 7 with Monday as 1 and Sunday as 7, no zero anywhere, and it's what business, banking, and most of the world outside North America run on.
The name is the same either way, Friday is Friday, but the number isn't. Feed a Sunday-based value into something that expects ISO, or the reverse, and you get a classic off-by-one that quietly shifts everything by a day before anyone notices Sunday and Monday traded places. Showing both side by side is there to spare you that guess when you're plugging a weekday into a formula or a schedule.
Why it agrees with itself in every time zone
The tool never asks your device what weekday a date is, it works the answer out from the bare year, month, and day using plain arithmetic. Build a date the naive way, hand a raw string to a generic date parser, and time zones creep in, a date near midnight can slide to the day before or after depending on where the browser thinks it's sitting, and your Saturday reads back as a Friday. Plenty of quick date tools ship exactly that bug and never catch it. Deriving the weekday straight from the numbers steps around all of that. You get the same answer in Tokyo, Toronto, or Auckland, because a calendar date doesn't carry a time zone and neither does the math here.
Frequently asked questions
Can it handle dates from hundreds of years ago?
Yes, the full range is year 1 through 9999. One honest caveat for very old dates: the tool uses the proleptic Gregorian calendar, meaning it applies today's leap-year rules even to years before the Gregorian reform of 1582, when much of the world was still on the Julian calendar. For a birthday or a modern historical event that's exactly what you want. For a genuinely ancient date, the weekday people wrote down at the time may not match, because they were counting on a different calendar. Worth keeping in mind if you're checking something medieval.
What's the difference between the occurrence number and a "last Friday" rule?
The occurrence counts forward from the start of the month, so a date can be the 1st, 2nd, 3rd, 4th, or occasionally 5th of its weekday. A "last Friday of the month" rule is a different animal. The last Friday might be the 4th or the 5th depending on the month, so don't read the occurrence as "last" without checking. If your rule is "last Thursday," look at whether a 5th Thursday even exists that month before you assume the 4th is the one.
Why does the ISO week sometimes show a year in parentheses?
Because ISO weeks don't line up cleanly with calendar years. Week 1 is defined as the week that holds the year's first Thursday, and whole weeks stay intact rather than getting split at January 1. So the first day or two of January can belong to the previous year's final week, and the last days of December can fall inside next year's week 1. When that happens the tool prints the ISO year in parentheses, so a January date reading as "week 52" makes sense instead of looking like a bug.
How many years until my birthday lands on the same weekday again?
It isn't a tidy seven, because leap years keep knocking the drift around. The gap between repeats of your birth weekday is usually 6 or 11 years, sometimes another spacing, depending on how the February 29s fall relative to your date. The years table shows it plainly, find the next row that matches your birth weekday and count the gap.
Does it tell me the day of the year and days remaining too?
It does. Next to the weekday you get the ordinal day (January 1 is 1, December 31 is 365 or 366) and how many days are left in the year. That's useful for anyone working with day-of-year numbering, some logistics and scientific and older mainframe systems label dates that way, and working it out from a wall calendar is a slog.
Is anything I enter uploaded or saved?
No. Every calculation runs locally in your browser with simple arithmetic, nothing is sent to a server, logged, or stored. The dates you type stay on your device, and the tool keeps working even if you drop your connection.