Odd Week, Even Week Custody Schedule: How to Count It Right, and the 2026 Trap
Almost every shared custody arrangement talks about odd weeks and even weeks. Here's how to count them without getting it wrong, why 2026 breaks the usual alternation right at Christmas, and what to write down or double check so both parents are looking at the same calendar.
Publié le 9 août 2026
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"The kids stay with Dad on even weeks and with Mom on odd weeks." That sentence, or something very close to it, shows up in most parenting plans and custody orders that split time week by week. It sounds perfectly clear. And yet, every year, it produces tens of thousands of "wait, is this an odd week or an even week?" texts, and at least one real argument, almost always in the same spot on the calendar: the turn of the year.
Who decides whether a week is odd or even
Nobody decides it: it comes from a standard, ISO 8601, the same one used by businesses, schools and every electronic calendar. It comes down to two rules.
- A week starts on Monday and ends on Sunday.
- Week 1 of a year is the week that contains that year's first Thursday, which is another way of saying: the week that contains at least four days of the new year.
The Thursday rule is the one everybody forgets, and it's the one that matters. It's why a week that straddles two years always belongs entirely to one of them, never to both. Week 1 of 2026, for instance, started on Monday, December 29, 2025: it contains Thursday, January 1, so it belongs to 2026, even though three of its days are still in 2025.
On a phone, no calendar app shows week numbers by default. In Google
Calendar, the option is in the display settings; on an iPhone, it's
under Settings, then Calendar. A spreadsheet answers just as well:
=ISOWEEKNUM(TODAY()).
The trap: 2026 has 53 weeks
An ISO year has 52 weeks, except when it has 53. That happens when January 1 falls on a Thursday, or on a Wednesday in a leap year. 2026 is one of those years: January 1, 2026 is a Thursday. The other years it happens around us are 2020, 2026, 2032, 2037 and 2043.
And here's the consequence, which is anything but theoretical:
| Week | From Monday | To Sunday | Parity |
|---|---|---|---|
| Week 52, 2026 | December 21, 2026 | December 27, 2026 | even |
| Week 53, 2026 | December 28, 2026 | January 3, 2027 | odd |
| Week 1, 2027 | January 4, 2027 | January 10, 2027 | odd |
| Week 2, 2027 | January 11, 2027 | January 17, 2027 | even |
Two odd weeks land back to back. A parent who has the odd weeks keeps the kids from December 28, 2026 through January 10, 2027, fourteen nights in a row, while the other parent gets none of that stretch. It's not a mistake, it's the literal result of the alternating pattern. And it's also, every single time, the moment one parent discovers this the night before and assumes the other one is cheating the schedule.
The same thing happened at the end of 2020, and it will happen again at the end of 2032. It always lands in the worst possible spot: the winter break, the exact stretch most custody orders already carve out with their own holiday clause.
What you can do about it, in order of preference
- Check what your custody order says about school breaks. In most cases, the holiday clause overrides the odd/even alternation for the whole length of the break, and the problem simply disappears. That's the first thing to check, before any conversation happens.
- Agree on a make-up swap, in writing, in November rather than December. A text or an email is enough: what matters is that it's dated and both parents have it in front of them.
- Do nothing, and let the fourteen nights happen, noting that they'll even out later. That's a perfectly valid answer, as long as it's a choice and not a surprise.
Even for whom? The order doesn't always say
A custody order can simply say "alternating, one week on, one week off" without specifying which parent gets which parity. When that's the case, the date of the very first handover sets the pattern for good, and it's worth writing it down the day it happens: three years later, nobody remembers it for certain anymore.
One thing worth saying clearly: "even weeks for Dad" isn't a rule, a custom, or something courts lean toward. It's a split decided purely by which week the schedule happened to start on, and the two halves are strictly equivalent. Over a full year, odd and even weeks share out holidays and long weekends in a comparable way, and any gap gets corrected by the holiday clauses anyway.
The handover day matters as much as the week
An ISO week starts on Monday, but almost no custody schedule switches over on a Monday. The two most common patterns are:
- Friday after school to the following Friday, by far the most common: the child switches homes at the end of the school week, and the weekend follows whichever parent they're arriving with;
- Sunday evening to Sunday evening, which keeps full school weeks together and splits the weekends instead.
With a Friday handover, the "even week" actually starts on the Friday of the odd week. That three-day offset is the second biggest source of confusion, a distant second to the new year issue but well ahead of everything else. It's worth writing down once, clearly: "the kids come to X's on the Friday before the even week starts."
A printed calendar only holds up for so long
A paper chart with pencil marks does the job for the first few months. It struggles after that, for three reasons that show up in every family: school breaks don't follow the odd/even pattern, one-off swaps (a wedding, a work trip, a bout of chickenpox) never get written down on both sides, and six months in, nobody can say with certainty how many nights each parent actually had.
That's exactly what Nestido does for you: you declare your schedule once, the app fills in the months ahead, and all you have to note are the exceptions. Week numbers, 2026's 53 weeks, the school holidays for your zone and the public holidays are already built in, and the other parent sees the exact same calendar at the exact same time.
To go further: which custody schedule to choose, how to split school holidays and who has the kids for Christmas.