Does a habit have to be every day?

No. What the research asks for is a stable cue and enough repetition, not a daily one. The thing that actually predicts failure is not a lighter schedule — it is a schedule you never chose, where some weeks are three times and some are none.

what the research actually requires

The 2009 University College London study that gave us the number 66 asked volunteers to do one simple thing in the same situation each day. Two findings matter here, and they point in opposite directions from the usual advice:

Missing a single day did not measurably set anyone back.
People who performed the behaviour inconsistently, week after week, did not reach automatic at all.

So repetition matters and regularity matters, but “every single day” was never the finding. More on what that study found.

the difference that decides it

Three times a week on purpose and three times a week by drift look identical in a spreadsheet and are not the same behaviour at all. The first has a cue — Monday, Wednesday, Friday, after work. The second has no cue, so every session needs a fresh decision, and the decision is the thing habit formation is supposed to remove.

This is also why “I will go more often” is a weak plan and “I go on Tuesdays and Thursdays” is a strong one, even though the second asks for less.

how RIPEN handles a non-daily habit

RIPEN draws your habits as one tree, and each habit carries its own schedule. You can set named days — mon wed fri, weekdays — or “N days a week”.

Days you did not schedule cannot be missed. They are not in that habit’s calendar at all, so nothing falls on them.
A fruit grows when a branch holds 66 leaves at once, and leaves come from the days you did schedule. A habit three days a week reaches 66 in 22 weeks; five days a week, in about 13; every day, in 66 days.
Changing a schedule only works forwards. The Saturdays you marked for six months stay marked even after you move the habit to weekdays — the branch does not get shorter because you changed your mind about the future.
For illness or travel there is a break: up to 7 days off for every habit, and those days do not enter the calendar either.

where this leaves the daily habit

Daily is still the fastest route to a fruit and the easiest cue to keep, because “every day” needs no remembering. But it is a choice about pace, not a requirement for the habit to form — and a schedule you can actually hold beats a daily one you abandon in March.

No account, no analytics, and Pro is bought once rather than every month. How the tree works, or get the app.

sources

Lally, P., van Jaarsveld, C. H. M., Potts, H. W. W., & Wardle, J. (2010). “How are habits formed: Modelling habit formation in the real world.” European Journal of Social Psychology, 40(6), 998–1009. The schedule arithmetic above is RIPEN’s own, from the rules in the app.