21 June 2025
What UK product teams ask about retention windows
Seven-day retention is a convention imported with the dashboard, not a fact about British weeks. Students in Llandow ask about bank holidays before they ask about Bayesian shrinkage, and they are right to.
A grocery app’s “week” is not a fintech app’s week. If your users shop after payday, a window that starts on a random Tuesday will wobble. We teach people to publish the window next to the number — D6 for a Sunday-first shopping habit is not a scandal; an unexplained D7 copied from a US consumer social app might be.
The calendar is an event property
Annotate English and Welsh bank holidays, and do not assume Scotland and Northern Ireland share them. A cohort that installs on the Friday before a late-May holiday will look sticky on Monday for reasons that have nothing to do with onboarding copy. Store both install_date and a local_calendar_bucket if the app is UK-only. If you ship further, nation is not optional decoration; it is how a KYC funnel once hid a Wales-shaped hole in our case notes.
Store timing
App Store review lag means a feature can “ship” on a Thursday in engineering and land for users on a Tuesday. Retention windows that start at git-merge time will smear the class. Start the clock at first_open after the build is live, and write that sentence in the dictionary.
Privacy, briefly
Retention is still first-party measurement. Teams ask whether a longer window requires a new legal basis. A window is not a new identifier. What does require care is joining retention to advertising click IDs — work we refuse in the atelier. Read the privacy notice for how we handle student data; your own app’s notice is a different document and should be written by people who have read the ICO’s guidance, not by a tracking-plan template.
If you want a default: pick the shortest window in which a second intent can honestly occur, publish it, and stop changing it every quarter so that graphs can be compared. Vanity is often just restlessness.