Automatic, or yours
By default your check-in day is Automatic: the weekday you created your account, which spreads check-ins naturally across users. If a different day fits your life better — Sunday evening planning, Monday fresh starts — pick it explicitly under Program → Settings → Check-in day.
Which day is best? The one where you'll actually sit with it for two minutes. Just before your grocery shop is a popular choice.
What happens when you change it
The change takes effect from the next day onward — switching your check-in day to Monday on a Monday doesn't conjure an instant check-in; the first eligible one is the next occurrence after at least a full day has passed. This keeps the reviewed week meaningful rather than letting a settings change manufacture a review on the spot.
The usual rhythm rules still apply on top: about a week since your last check-in, and a program at least a week old.
The countdown chip
Your program's targets card counts it down for you — "Check-in in 3 days", "Check-in tomorrow", and finally "Check-in ready", which is tappable and takes you straight into the check-in. Miss the day? There's a grace window of a few days after it before the chance rolls to the next week.