How to capture Event date to update a custom field

How to capture Event date to update a custom field

I have created a custom field in Accounts called "Last Visit Date." I would like to update this field automatically every time a new Event is created. I created a Workflow rule that is executed any time a new Event is created with a related Action to update the custom field. The only field update options I have are ${EXECUTION_DAY} or a date picked from the calendar. How do I capture the Event "From" date, not the date the Event was created? Any suggestions? Is this even possible?