You come across a task on your calendar, and since you are not in the same mindset as when you scheduled it, you may forget the importance of the task and postpone it or ignore it altogether. This is an extremely easy problem to identify, but a much harder problem to correct.
Update: Ruben from the comments brought up a great point. This is most crucial for dates that are far enough into the future that you could forget the importance.
A Possible Solution
Define the importance of every item on your to-do list.
Whenever you mark your calendar with a to-do item, put a small blurb about why it is important. These can include phrases that inspired you to take on the task, reasons why it is important, or anything else to jolt yourself back into the mindset surrounding that to-do item.
Remember, your past and present selves may not always have the same point of view.