Fasting is one of the traditional Catholic Lenten practices. All Catholics abstain from meat on Fridays during Lent as a form of fasting. For personal fasting, you can choose to voluntarily abstain from meat one more day during the week.
Consider abstaining on Mondays also and participating in the Meatless Monday movement. Skipping meat one day per week is also good for the environment, as meat requires more resources to produce than plant based foods.
It is important that abstaining from meat on Mondays not be seen as a substitute for abstaining from meat on Fridays during Lent. This should be “in addition to” and not “instead of”. Friday is a solemn day during Lent, because our Lord Jesus Christ gave his life for us on a Friday. Therefore Catholics should not feel like they can just swap out Fridays for Mondays. If you want to do meatless Monday, do both days!