The "Two-Day Rule" states that you should never skip the habit you're trying to build for two days in a row. The idea is that while it's easy to miss a day due to various reasons, skipping two days in a row makes it much more likely that you'll lose the habit altogether.

For example, if you're trying to establish a regular exercise routine and you miss a day, the Two-Day Rule would dictate that you absolutely must exercise the next day to keep the habit alive. Missing one day is considered an accident, but missing two days is seen as the start of a new habit—of not doing the activity.

This rule is effective because it strikes a balance between being strict enough to establish a routine and flexible enough to be sustainable. It allows for life's inevitable interruptions without letting you off the hook completely.