You have to remember that “when I was growing up” goes back to the late 50’s and predates the Brewers by over a decade. Doubleheaders were much more common then, and seemed to be as much the norm as the exception on Sundays. We attended a lot of Braves doubleheaders on Sundays. Gradually over the years the number of doubleheaders dwindled to the point that scheduled doubleheaders went the route of double features at the movies.
Maybe my recollection is fuzzy, but in those days I also recall a lot of holiday doubleheaders. One specific memory is going to the original Yankee Stadium on Tuesday, July 4, 1961 for a DH between the Yankees and Tigers, who were locked in a tight battle for first place. The place was absolutely packed with a crowd of over 70,000 as the teams split the pair and Roger Maris hit one of his 61 HR.
I looked up the date on Baseball Reference and on that Tuesday 10 of the 18 major league teams played in doubleheaders and the other 8 played in single games. That was pretty obviously a holiday phenomenon.
On Wednesday, July 4, 1962, the 20 major league teams played 8 doubleheaders.
On Memorial Day, 1961, which was on a Tuesday, there were 6 doubleheaders.
I didn’t look up any more dates, but there clearly is evidence that holiday doubleheaders were a thing in those days, even on midweek days.