Seems most opinions here tend towards getting the CFA done sooner rather than later, and before the congressional interviews. I am going to take the opposite view. If you believe you will score better by waiting, then wait as long as you are comfortable waiting to maximize your score while not missing the deadline. My son went through all of his nomination interviews with the exception of one that was later without having completed the CFA. He was asked about the CFA in all of his interviews and he explained what his training program was and when he planned to take it. Those offices said good luck. Notably, completing the CFA was not a requirement for the nomination packet, but I know some offices do have that requirement.
He waited till late fall because the weather was better than cranking it out in the heat. The extra two months he set aside for training dramatically improved his performance because he trained on the requirements nearly every single day, along with his normal weight training and running. This was especially important for improving the basketball throw and establishing proper techniques for the push-ups. I say he dramatically improved and that was based on me giving him a baseline test in the middle of the summer and then periodically taking practice tests along the way. When we watched him throw that basketball for record, we knew he made the right decision to wait.
He is a plebe now.