USCIS requires Affidavits:
Notarized affidavits of friends, neighbors, or other knowledgeable associates. Those who provide sworn or affirmed affidavits must have been living at the time the events occurred and have personal knowledge of the event that is to be proven.
Due to my Dad lost his marriage certificate, my dad and mom was married in 1957. There is no way to get his marriage certificate now. He needs to get 5 Affidavits from his old friends who attended his wedding in 1957. What is the format of the Affidavits?