Final answer:
To keep the original renewal month, you must start the CFI renewal process within 3 calendar months prior to the expiration date.
Step-by-step explanation:
The CFI certificate needs to be renewed every 24 months. To keep the original renewal month, you must start the renewal process within 3 calendar months prior to the expiration date. In this case, since the CFI certificate expires on December 31st, 2018, the earliest you could start the renewal process is September 1st, 2018, if you want to keep the original renewal month.