🎂How long until the anniversary?

The question is rarely "how many days". It is "is it on a weekend this year", and that changes every year.

Try one of these too

The weekday walk, and why it is not steady

A date does not keep its weekday, and the reason is a remainder. A common year is 365 days: 52 whole weeks with one day left over, so every date moves forward one weekday. A leap year is 366, which is two days left over, so every date after 29 February moves forward two.

Read the third row and the awkwardness becomes obvious: four years moves a date five weekdays, not four, which is why “it was a Saturday four years ago so it should be a Wednesday now” is wrong about as often as it is right. The fourth row is the one that closes the loop — 28 years is four leap cycles and exactly 1,461 weeks, so the whole calendar repeats.

That 28-year return holds inside a century and breaks at the ones that are not leap years. 1900 and 2100 skip their leap day, so any span reaching across them loses a day and the pattern restarts.

Weekend years come in pairs, and then not at all

Two days in seven are weekend days, so an annual date lands at a weekend roughly two years in seven. The spacing, though, is nothing like even.

Because a leap day makes the weekday jump two, a date that falls on a Saturday one year and has a 29 February immediately after it lands on a Monday — skipping the Sunday entirely. But a date on a Friday in that situation jumps to a Sunday, and a date on a Saturday in a year with no leap day in between moves to a Sunday too. The consequence is that weekend years arrive in consecutive pairs and then vanish for four or five years running.

For anything being planned a decade out — a fiftieth, a twenty-fifth, a reunion — that pairing is the useful fact. It is the difference between choosing which of two adjacent years to hold something in, and discovering that the only weekend option was three years ago.

29 February, and what the planner refuses to invent

A date that exists in one year out of four is the one case where a planner has to decide something rather than compute it. This one keeps the real date and lists only the years it occurs in.

The alternative — substituting 28 February or 1 March in the other three years — would produce a list of dates that look computed and are not. Which of the two substitutes is correct is a genuinely open question that different legal systems answer differently, and picking one silently would be presenting an opinion as arithmetic. Anyone who marks a leapling’s birthday on 28 February can read the leap years off the list and fill in the rest with a rule they have chosen themselves.

What to do with ten years of dates

The reason to look a decade ahead at an annual date is almost never the next one. It is to find the year that works: the Saturday, the one that does not collide with a school term, the milestone that happens to fall somewhere convenient.

For the arithmetic underneath — which years share an entire calendar, so that every date in them falls on the same weekday — the same calendar years tool shows the pattern and where it breaks. For a single date rather than a series, the day of the week calculator names the weekday for any date in any century. And for a one-off countdown that can be shared with the people it concerns, the custom countdown carries the date and the name in the link itself, with no account needed at either end.