When I was a kid, I heard someone say that April fools was a pagan holiday that meant to call Christians fools. However, from some research it appears that it has nothing to do with God or theology. Evidently, back in 1564 King Charles IV of France introduced a new calendar. Up until then, the new year had been celebrated from March 21st until April 1st, but Charles' new calendar made January 1st the new New Year's Day. Some people didn't adjust to the change and kept celebrating April 1st. They came to be known as April Fools.
Nowdays, I would probably call those who say there is no God is a fool. "The fool says in his heart, “There is no God.”Psalm 14:1. No God, no hope. May we celebrate having a God whose grace is above all even to the cross.