Lets say I have 1000 records

I add a field ' Week of year'

I have a field called  Visit_date which has an  on user input  Script that reads ' Week_of_year = Visit_date.getWeekOfYear()'

Script works fine

My problem is how do I populate the 'Week of year field' without going into every record and activating the script in the field by changing the date.

Thanks