It is my understanding that Sundays--even the Sundays of Lent--are not penitential days; they are always celebratory days of rejoicing. So if you've given up sweets for Lent, you can indulge some on Sundays, even during this penitential season. If I further recall correctly, this is why Sundays aren't counted at all as the days "of Lent." If you go by the calendar, Lent seems to last for 46 days--if you count the Sundays. But we don't include those Sundays, so Lent consists of 40 days.