Birthday weekday calculator
Someone born on 25 July 1990 has their 2026 birthday on a Saturday. Enter your birth date and the year you are planning to see which weekday the anniversary lands on, with 29 February checked against real leap years. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Date
- 1990-07-25
- Year
- 2,026
- Month
- 7
- Day
- 25
- Birthday
- 2026-07-25
- Weekday
- Saturday
This birthday weekday calculator answers a question people type every planning season: what day of the week does my birthday fall on in a given year? You enter a birth date as an ISO calendar day and a target year. The engine copies the month and day onto that year, checks that the anniversary is a real Gregorian date, and returns the English weekday name. February 29 is accepted only when both the birth year and the target year are leap years, so a twenty-ninth of February in a common year is rejected instead of silently sliding to the twenty-eighth or the first of March. Use it when you already know the birth record and only need the weekday of that anniversary in another year.
What a birthday weekday is and how the anniversary is built
A birthday weekday is the day of the week on which the anniversary of a birth date lands in a chosen year, not the weekday of the original birth itself unless that year is the one you asked for. People open this landing when they want to know whether a coming birthday is a Saturday they can host on, whether a childhood birthday was a school day, or which weekday a famous birth will occupy on a centenary.
The inputs are a birth date written as an ISO calendar day and a target year written as a Gregorian year from one through nine thousand nine hundred ninety-nine. The engine copies the month and the day of the month from the birth date onto the target year, checks that the resulting anniversary is a real civil date, and names the weekday in English.
That split is the product: you do not send a complete date for the year you care about; you send the birth record plus the year you are planning. February 29 is the only anniversary that can vanish. A person born on the twenty-ninth of February has a calendar birthday only in Gregorian leap years.
Invalid birth dates, including the twenty-ninth of February in a non-leap birth year, the thirty-first of April, or a string that is not year-month-day, also fail. Leap years follow the usual rule: divisible by four, except century years that are not divisible by four hundred.
Where a birthday weekday calculator earns its keep
Households and event planners use a birthday weekday calculator so they can book a venue before they print invitations, because a Saturday anniversary is a different shopping list from a Tuesday. Teachers and exam-prep sites need a deterministic oracle that marks a twenty-ninth of February in a common year as invalid instead of silently moving the date, which is a favourite trick question in calendar homework. Genealogy pages and sports almanacs reconstruct the weekday of a historical birth in a later commemorative year without depending on the host clock.
Product teams that already ship a generic day-of-the-week tool still need this landing: searchers type what day of the week is my birthday, not what weekday is this ISO date, and they arrive with a birth record plus a year rather than a complete target date. Failed validation does not consume balance. Keep requests small, pin the weekday string in tests, and you get a reproducible anniversary name for homework, LMS plugins, and edge workers without standing up a calendar library of your own.
This page is not an age calculator, not a birthday-paradox probability tool, and not a locale-aware date formatter.
What you can do with it
Plan a party around the weekday
Enter a date of birth and the year you are hosting to see whether the anniversary is a Saturday, a school night, or a public-holiday weekend before you book a room.
Homework and calendar exams
Check the weekday of a textbook birthday in a later year, including leap-day failures, against a deterministic oracle instead of a timezone-sensitive Date call.
FAQ
How do I find what day of the week my birthday falls on?
Send the birth date as YYYY-MM-DD in date and the year you care about in year.
What counts as an invalid birth date?
Anything that is not a real Gregorian YYYY-MM-DD day: wrong shape, month 00 or 13, 31 April, or 29 February in a non-leap birth year. Those requests are not billed.
What if I was born on 29 February?
The birthday is accepted only when the target year is also a leap year. In a common year that date does not exist, so the calculator says so instead of quietly moving it to the last day of February or to 1 March.
Is this the weekday I was born on?
Only if you choose your birth year as the target year. Otherwise it is the weekday of your birthday in the year you pick, such as 25 July 1990 landing on a Saturday in 2026.
Which years can I check?
Any Gregorian year from 1 through 9999. Leap years follow the usual rule: divisible by four, except century years that are not divisible by four hundred.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/life/birthday \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"date":"1990-07-25","year":2026}'const res = await fetch("https://api.kit.forhosting.com/life/birthday", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"date": "1990-07-25",
"year": 2026
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/life/birthday",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"date": "1990-07-25",
"year": 2026
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/life/birthday", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"date":"1990-07-25","year":2026}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"date":"1990-07-25","year":2026}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/life/birthday", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"date": "1990-07-25",
"year": 2026
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "life.birthday",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
year_min | 1 |
year_max | 9999 |
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
What a birthday weekday is and how the anniversary is built
If the birth date is the twenty-ninth of February in a leap year and the target year is a common year, this calculator rejects the request instead of inventing a legal convention such as celebrating on the twenty-eighth of February or the first of March.
Input contract, response shape, and error behaviour
Failed validation is never billed. Typical failures are a missing date or year, a birth string that is not year-month-day, an impossible civil day such as 2023-02-29, a year outside the documented bounds, and a twenty-ninth of February projected onto a common target year. POST a JSON body with two required fields. The birth date lives in date and may also arrive as birth_date, birth, dob, born, or birthdate. It must be a string that matches four digits, a hyphen, two digits, a hyphen, and two digits, and it must name a real Gregorian day in the inclusive year range from one through nine thousand nine hundred ninety-nine. The target year lives in year and may also arrive as target_year or in_year. It may be a JSON integer or a whole-number string; fractions, empty strings, and values outside that year range fail with invalid_input that names the field. On success the body echoes the canonical birth date, the target year, the month and day of the anniversary, the ISO anniversary string in birthday, the English weekday name, a three-letter weekday_short, the ISO weekday number where Monday is one and Sunday is seven, a leap_year flag for the target year, and a short message such as Birthday 25 July falls on Saturday in 2026. Pricing is a flat $0.002 per successful request with unit item. Pair the response with your own calendar UI if you need a month grid; this endpoint names the weekday only and does not emit locales, time zones, or party-planning checklists.
Integration use cases
Invitation and genealogy fixtures
Seed test data and commemorative pages with the English weekday of a known birth date projected onto a chosen year, without reading the host clock.
Technical questions
What happens if I was born on 29 February and the target year is not a leap year?
The task fails with invalid_input. This calculator does not move a 29 February birthday to 28 February or 1 March; the anniversary must exist on the civil calendar.
Does the weekday depend on my timezone?
No. The solver uses Sakamoto Gregorian arithmetic and never reads the host clock or the Date timezone, so the browser tool and the API agree.
What does the birthday weekday calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.