You're staring at an old receipt. Or a birth certificate. Also, maybe a court document, a journal entry, or a photo with a scribbled date on the back. Still, may 10, 2010. You need to know what day of the week it fell on — and you need to be sure Less friction, more output..
Here's the short answer: May 10, 2010 was a Monday.
But if you're here, you probably want more than just the answer. You want to know how to verify it, why it matters, and how to figure out any date without guessing. Let's get into it.
What Day Was May 10, 2010?
Monday. First full workweek of May that year. The 130th day of 2010 (131st in leap years — but 2010 wasn't one). 235 days left until New Year's Eve Not complicated — just consistent..
If you're doing the math in your head: January 1, 2010 was a Friday. Friday + 4 days = Tuesday? Because January 1 counts as day 1, not day 0. Wait — that's off by one. 129 ÷ 7 = 18 weeks, 3 days. But add the days through April (31 + 28 + 31 + 30 = 120), plus 10 days into May = 130 days total. So 129 days after Jan 1. Friday + 3 = Monday. Still, 130 ÷ 7 = 18 weeks and 4 days. There it is.
We'll come back to the math. First — why does anyone care?
Why Would Anyone Need to Know This?
More often than you'd think.
Legal and financial deadlines
Statutes of limitations. Contract clauses. "Within 30 business days of May 10, 2010." If you're a paralegal, an accountant, or just someone disputing a charge, the day of the week changes the count. Monday means the clock starts ticking that day. Sunday? You'd start Tuesday Most people skip this — try not to. Worth knowing..
Genealogy and family history
You found a letter dated "May 10, 2010 — Mom's birthday." Was it actually her birthday? (It wasn't — Mother's Day was May 9 that year. The 10th was the Monday after.) Knowing the day helps verify stories, match photos to events, place a diary entry in context Worth keeping that in mind..
Scheduling and planning
Ever tried to recreate a timeline? "We launched the product the Monday after Mother's Day 2010." That's May 10. Now you're building a case study, a resume, a portfolio — and you need the date to be precise.
Simple curiosity
Sometimes you just wonder. What day was I born? What day did we get married? What day did the world change? May 10, 2010 didn't change the world — but it was someone's world-changing day Most people skip this — try not to..
How to Figure Out Any Date's Day of the Week
You don't need to memorize calendars. You need a method. Here are the ones that actually work.
The Doomsday Rule (John Conway's method)
This is the gold standard for mental calculation. Learn it once, use it forever.
Core idea: Certain "anchor dates" in every year fall on the same day of the week — the "Doomsday." For 2010, Doomsday was Sunday And it works..
The anchor dates (same every year):
- 4/4, 6/6, 8/8, 10/10, 12/12
- 5/9, 9/5, 7/11, 11/7 (the "9-5 at 7-11" mnemonic)
- Last day of February (Feb 28 or 29)
- January 3 (or Jan 4 in leap years)
- March 0 (i.e., last day of February)
So in 2010:
- 10/10 was a Sunday
- 5/9 was a Sunday
- 9/5 was a Sunday
- etc.
May 10 is one day after May 9. So Monday. Done Not complicated — just consistent. That's the whole idea..
To find any year's Doomsday:
- Take the last two digits of the year (10 for 2010)
- Add the number of 12s in that number (0 for 10)
- Add the remainder (10)
- Add the number of 4s in the remainder (2)
- Sum: 10 + 0 + 10 + 2 = 22
- 22 mod 7 = 1
- Anchor day for 2000s = Tuesday
- Tuesday + 1 = Wednesday? Wait — that's for 2010? No. Let me recalculate.
Actually, the century anchor for 2000–2099 is Tuesday.
Year offset for 2010:
y = 10
y + ⌊y/4⌋ = 10 + 2 = 12
12 mod 7 = 5
Tuesday + 5 = Sunday. Which means yes. Doomsday 2010 = Sunday.
May 9 = Sunday → May 10 = Monday And it works..
It works. Think about it: every time. Once you practice it, you can do any date in 10 seconds.
Zeller's Congruence
The formula programmers use. For Gregorian calendar:
h = (q + ⌊13(m+1)/5⌋ + K + ⌊K/4⌋ + ⌊J/4⌋ + 5J) mod 7
Where:
- h = day of week (0 = Saturday, 1 = Sunday, 2 = Monday... That's why 6 = Friday)
- q = day of month (10)
- m = month (3 = March, ... , 14 = February?
For May 10, 2010:
- q = 10
- m = 5
- Year = 2010 → K = 10, J = 20
- h = (10 + ⌊13×6/
