Ok, @JohnT and others, I just had a lady I work with (a very devout Catholic) tell me she could partake of her lenten sacrifice on Sundays. I told her "no you can't, it's 40 days of fasting". So after a little interweb research, this appears.
Please do explain to grasshopper.
As the "no cheating" folks like to point out, Jesus did not come in from the desert on Sundays. But, here's the thing. When Jesus was in the desert, Sundays weren't Sundays yet. When Jesus went to the desert, he had not yet begun his ministry, he had not yet suffered his passion, he had not yet died for our sins and risen again on the third day. But once that very first Easter happened, Sundays became something set apart, something special. Each and every one is a feast.
The very earliest Christians recognized that. Medieval Christians were on board with it. Pre-Vatican II types knew a Sunday when they saw one, even in Lent. That's why, when you count up the days on the calendar, the season of Lent is NOT 40 days, it's 46 days. Because, from the very beginning of Lents, Sundays were not included as part of the Lenten fast.
It's not that fasting isn't REQUIRED on Sundays and Holy Days of Obligation in Lent. It's actually FORBIDDEN.
Now, most of us do not fast from food except as required on Ash Wednesday and Good Friday. Most of us choose to fast from something else, like sweets or television, instead. But that's still a fast. And fasts don't apply on Sundays.
We're not required to have sweets or watch TV on any particular Sunday, of course, so we're not required to seek them out on Sundays during Lent either. But if the opportunity arises to partake in a licit pleasurable activity on a Sunday during Lent, we shouldn't refrain because of our Lenten disciplines. Every Sunday is a little Easter. Every Sunday should feel like a celebration. If your Lenten discipline is a real, prayerful struggle, that's great. But Sunday isn't the place to struggle against licit pleasures.