An outside day is a daily price action that has a higher high and lower low than the previous price bar.
The outside day also has an open and a close that goes beyond the previous open and close.
When price bars move in opposite directions, this is called an outside reversal.
Context is important when trading off-days: it includes the volume, the overall direction of the trend, the direction of the price bars within the outside day pattern, and the direction of the price bar following the pattern.