Has it every felt like your birthday seems to always fall on a particular day of the week? Have you every thought, ‘Gee, I never seem to have my birthday on a Saturday, so that I can throw a big party!’.
That sentiment is likely all in your head, or you haven’t lived long enough, as over the years, your birthday will occur fairly evenly over all the weekdays.
I built a Birthday Weekday Calculator to prove it.
It turns out that there’s a an explanation which, once you hear it, makes a lof sense. A regular year has 365 days - that’s 52 weeks plus one extra day. That one leftover day will mean your birthday shifts forward by exactly one weekday each non-leap year. If your birthday was a Thursday last year, it’s a Friday this year.
So what happens on a leap year? In a leap year, you get two extra days instead of one - so you jump two weekdays instead of one.
The full pattern repeats every 28 years. Same date, same weekday - 28 years later. Once I learned that, I wanted to actually see it play out across every birthday I’ve had.
That’s what this calculator does. You enter your birthday and it tells you:
- What day of the week you were actually born on
- What day your most recent birthday fell on
- What day your next birthday will fall on
- And then the fun part - a full visual chart showing every birthday you’ve had, grouped by weekday
I’m curious if you find the chart interesting. You can see your entire birthday history arranged into columns by day of the week. Some years cluster up in certain columns, then shift, then come back. You can actually see the 28-year rhythm playing out if you’re old enough.
Give it a try at the Birthday Weekday Calculator. I’d be curious whether you were born on a weekend or a weekday - and whether your recent birthdays have been kind to you on that front.
I’m on Bluesky if you want to share what you find.