If you set the rate at a later date, such as when the loan is approved or a few days prior to closing, you would use the index on that date.
I just want everyone to be clear on the "index" and the date.
When you set your rate you use the 5-year constant maturity as your index then you can use any value for that index within the past 45 days, providing your contract calls for the 45 day look back.
If you use the 5-year consant maturity value in affect on 11/11 but you set the rate on 12/8 the APOR "index" in affect on 12/8 is the APOR index the APR is compared to. It will not be copared to the APOR index in affect as of 11/11.