When it comes to the Sundays of the church, most have the title "# Sunday of ________". Exceptions come in the form of the Sundays that follow Epiphany, each listed as a "Sunday after the Epiphany", and Pentecost, each listed as a "Sunday after Pentecost".
Lent is a different. In this season, the Sundays aren't each listed as a "Sunday of Lent" but instead as a "Sunday in Lent".
Lent is a time of fasting, as we hear in the service for Ash Wednesday. However, Sundays aren't. Sunday is always a feast day, specifically a feast day of the Resurrection of our Lord Jesus Christ. That is why the Sundays in Lent aren't of Lent. They aren't part of the Lenten fast. They are pauses during our fasting. In fact, if you look closely at the calendar from Ash Wednesday to Easter, you will count 46 days, not 40. The Sundays in Lent (including Palm Sunday: the Sunday of the Passion) make up those 6 extra days.
If you've given anything up for Lent, this means it is okay for you to take a break on Sundays. Sunday is a reminder that even in times of fasting we have hope in our Lord Jesus Christ.