One of the big points of interest for Catholics is the idea that Sundays in Lent are "days off" from giving up whatever you gave up. If you gave up chocolate for Lent, you can have some on Sunday and just abstain from Monday through Saturday, inclusive. I've heard and read a lot of different arguments about when we have to offer something up and when we can indulge.
The crux of all the arguments is that Lent is supposed to be 40 days long, in imitation of the 40 days of fasting and preparation that the Lord did before He started His public ministry.
One argument is the number of days from Ash Wednesday to Easter Sunday is 49, so that in order to hit the magical "40 days" number, you can indulge on Sundays (of which there are six before Easter comes) and also on the Solemnities of St. Joseph (March 19) and the Annunciation of the Lord (March 25) and, if you are Irish, the greatest of all possible feasts, St. Patrick's Day (March 17, like I had to tell you!). So these are all days that you don't have to give up whatever it is you gave up.
Another argument is the number of days from Ash Wednesday to Easter Sunday is 40, so you have to tough it out until Easter sunrise. No breaks for Sundays, St. Joseph, the Incarnation or even St. Patrick (though the local bishop may give you St. Patrick's day off anyway, especially if it falls on a Friday in Lent--we all know that corned beef and cabbage is too tempting and that no true blooded Irishman or Irishlass can refrain on March 17, though the legitimacy of the dish is another controvery I don't want to get into here. Look here instead.)
Another argument is that Lent is 46 days long and just Sundays are off from penance. This is what I grew up with, along with the practice of being "free" from what you offered up at noon on Holy Saturday. I've never really understood this "Saturday noon" thing so I can offer no justification for it except tradition with a small t.
Taking a calendar and actually counting the number of days starting with Ash Wednesday and ending on Holy Saturday, the number is indeed 46. Since every Sunday is the celebration of the Resurrection of the Lord, it makes sense that it is not a penitential day. Though nothing stops someone from continuing a Lenten practice. Clearly, the church doesn't fully celebrate on Sundays in Lent, since the Gloria and the gospel alleluia is dropped for those six Sundays.
I think that obsessing about the number of days that you fast, pray and give alms is really not conducive to receiving the benefits of penitential, prayerful and sacrificial preparation for the resurrection of our Lord Jesus Christ. If you just want to fulfill the law or hit the right number, you aren't really coming closer to Christ, trying to understand His will for you and being open to His influences on your life. Saying "I did 40 days just like Jesus did, so look at me!" is like saying "I have a beard and mustache just like Jesus did, so look at me!" You really miss the point of conversion to Christ that we all should seek and that we all constantly have to renew.