Habitualist lets you specify a minimum and (optional) maximum frequency of each action in units of either days, weeks, months or even (by request) years. That means that you can say you want to do things “every 3–5 days” or “every two weeks” or “every 3–5 weeks” or “every 1–2 months”, etc. (An upcoming blog post will talk about use cases for some of these less frequent intervals.)
You might think that “every week” is the same as “every seven days”; that “every 12 months” would be the same as “every year” but Habitualist treats them differently. If you say “every week”, Habitualist interprets this to mean that if you've done it this week, you don't have to do it until the following week. So if you do it on a Tuesday one week and a Saturday the following week that's fine; and if you do it on a Saturday one week, doing it on the Monday two days later is fine. What matters is not so much the number of days between the occurrence as the fact they were in separate weeks. Saying “every seven days”, on the other hand, means that seven actual days should pass before you do it again. Do it on a Tuesday one week and you shouldn't do it until Tuesday the following week.
So if something should be done roughly once a week in the sense of roughly seven days passing between each occurrence, you might be best off saying “every seven days” or “every 6–8 days”. If you truly want to do it once per calendar week, “every week” is the way to go.