I think that there are various traditions concerning this, and it may have depended on the region you live in, or your ethnic background or something, because when I was growing up in the 50’s and 60’s, we always excluded Sundays in our Lenten fasts, and in my Catholic school we were taught that one did not have to fast on Sundays. That was the one day we could have a dessert, or candy, etc., although we were not to go overboard on it.
Of course, since we fasted from midnight on before taking Communion, that meant that you couldn’t wake up on Sunday morning and have a piece of cake for breakfast before Mass! And of course, there was no dessert or candy until you had eaten a decent dinner, around noon, so Sunday was half over before you got your sweeties. We were very deprived in those days.