Notifications should go off on the days that are due for your habit at the time you've set in your habit settings (the app's default is 6pm). Please note that a notification will not fire if you've already checked in for the current day earlier than the notification time.
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Also, please ensure that the app is allowed by iOS to send notifications. You can do that by opening your iPhone's settings -> Notifications and verify that the respective settings for Today are enabled.