Enrollment Override Thru Date questions

If I want to change a rate start date(when the deduction starts) for an employee, which enrollment override "thru date(s)" do I need to enter.  For example, if the Life Event starts the deduction on Jan. 1 but I would like it to start on Dec. 24, I realize that I have to change the start date on the "Enrollment Override --> Rates --> General --> Start" field.  Do I need to add a "Override Thru" date in this area and a "Thru" date on the "Enrollment --> Override --> Thru" field?  Also, what date is recommended to use?