If it is  prior to 3 pm on the day before the event:
Click My Calendar
Deselect the checkbox for the event. The box should turn white and the checkmark will be removed.
Click Save Month
If it is  after 3 pm on the day before the event:
Please try to avoid this situation, but stuff happens.
Step 2 above won't work. The checkbox will be grayed out and you will not be able to remove the checkmark.
Please email your league manager AND Phil Woodrum, to notify them that you must cancel.