My wife Heather calls her in-laws by in-loves. When you get married, the father of your spouse is your father-in-law (father-in-love). This day is held annually on July 30. I couldn’t find out when or how this day got started.
So Happy Father-in-Law Day to my Father-in-Love, Tim!
(Learn more about the term -In-Loves instead of In-Laws)