Started Dating on the 31st? When Your One-Month Monthsary Actually Falls

It is one of the most oddly specific questions couples actually argue about: if you start dating on the 31st, when is your one month? January has a 31st. February absolutely does not. So is your first monthsary on February 28th? March 1st? March 3rd, thirty days later? Somebody in this relationship wants to book a dinner, and the calendar is refusing to cooperate.

The honest answer is that there is no official rulebook — a monthsary is a tradition couples invented, so couples get to pick the rule. But there are three sensible conventions, each with its own logic, and this post lays them out so you two can pick one and never have this argument again.

A monthsary normally works by simple repetition: got together on the 12th, celebrate every 12th. That works perfectly for dates 1 through 28, because every month has them. Start on the 29th, 30th or 31st, though, and some months skip your date entirely. The 31st is missing from four months of the year (February, April, June, September, November make five short months, four of which have 30 days), and February skips the 29th and 30th too.

So if your anniversary date is the 31st, you have seven real 31sts a year, and you need a policy for the other five months. Here are the three policies couples actually use.

When a month does not have your date, celebrate on its last day. Started on January 31st? Your one-month monthsary lands on February 28th (or February 29th in a leap year), your March monthsary is back on the 31st, your April monthsary is the 30th, and so on.

This is the rule most couples settle on, and it is also how most date-counting apps and phone calendars handle recurring monthly events on the 31st. The logic: a monthsary marks the completion of a calendar month together, and February is complete when February ends. You never skip a month, the date never drifts, and every celebration still falls in the month it belongs to.

The alternative: if the month has no 31st, roll forward to the next real day — March 1st instead of February 31st. The logic here is about not celebrating early: on February 28th, the pedants argue, a full calendar month has not quite passed, so you wait until the calendar clicks over.

It works, but it has a quirk: your February monthsary now happens in March, and in leap years the whole question of February 29th versus March 1st reopens. Couples who choose this rule tend to be the ones who enjoy the debate more than the dinner.

The third approach ignores calendar dates entirely: a month is 30 days, so celebrate every 30 days from the day you got together. Your milestones stop being tied to the 31st at all — they land on a slowly rotating set of dates, and after a year you are about five days ahead of the calendar couples.

This rule is the least common for monthsaries but the most satisfying for people who love clean numbers, and it pairs naturally with day-based milestones — 100 days, 500 days, 1000 days together — which many couples track alongside or instead of monthsaries. If that is your style, our guide to the 100 days together milestone is written for you.

Short version, using the most popular last-day rule:

Every option above is defensible, which is exactly why the argument is pointless. Pick a rule as a couple, say it out loud once — 'we are last-day people' — and the problem is solved for the rest of the relationship. The milestone was never really about calendar arithmetic; it is about one month of showing up for each other, which you did, whatever February thinks.

And if you would rather never do date math again: CountingUs counts it for you. Set your start date once and our days-together counter always knows exactly how long you have been an us — days, months, milestones and all. If you are still choosing your celebration day, our guides to the best date to celebrate your monthsary and what a monthsary actually means are the natural next reads.