There are easy days and then there are those days where being calm under pressure and managing the tasks becomes extremely vital!
As a commercial airline pilot, I start to think about my flight a few days in advance based on factors such as the weather trend, industrial issues, operational restrictions, type of airfield I will be operating into, security threats, the crew composition (could be a new entrant First Officer with little or no commercial airline experience), possible technical defects with an aircraft and the flight routing.
Weather - in aviation this can turn out to be one of the most regular and largest threats when it comes to safety.
Monitoring weather trends and the predicted forecast for the flight allows us as pilots to better plan any contingency procedures we may need on that day. The weather analysis will include that for the departure/destination airfields including all possible alternates along the way. Phenomena such as thunderstorms, visibility, wind, wind-shears, sand-storms, turbulence, etc, can add significant stress and safety issues to an operation. Sometimes weather conditions could even be out of the aircraft's operational limits. Proper forecast and planning allows us to create better and safer alternate plans and carry the required extra fuel for a second approach and the best possible alternate in terms of weather and technical degradations.
Industrial issues and operational restrictions - although such events occur rarely, they can add to several other problems directly or indirectly related to the flight.