St. Joseph was, of course, a step-parent. I've not usually heard the term applied to him except in jest. But the spouse of Mary the Mother of God is surely the Step-Father of God. And in the case of Jesus, "You're not my real dad" would have a little extra heft to it.
The Gospel appointed for today in the Anglican Church of Canada is Luke's account of the 12 year old Jesus in the Temple. Let's face it, "Did you not know that I must be in my Father's house" was essentially a very polite "You aren't my real dad."
We don't know much about Joseph from the Gospel accounts, but we do know that he accepted the role of step-father, even though it would have been very easy - perhaps far easier - for him to have rejected it, to have stuck to his plan to quietly end his betrothal to Mary. But he didn't, serving as step-father, provider and protector to Our Lord in his infancy, childhood and adolescence.
So perhaps today is a day to remember the ministry and sacrifice of step-parents, whose patron St. Joseph really ought to be.