Because Habitualist lets you log actions at a weekly level as well as daily, the definition of a "week" is important. If you'd like a week to run from Monday thru Sunday, click on your username in the top right and select Account. As well as your email address and timezone, you can now select a week start day of Monday.
Be sure to set both your timezone and week start day preference for optimal use of Habitualist.
NOTE: you may get unexpected results if you change your week start preference after a weekly action has already been started. A action marked done for this week could change to being done for last week, for example, because the most recent Sunday is no longer this week.