The words, Solis et Veneris, are the Latin words for Sunday and Friday.
Sunday, being the day Jesus resurrected, is a day to rejoice. Friday, being the day He died, is a day to be mortified. For these reasons, we feast on Sundays and fast on Fridays.
Both are to be done in moderation and with right intention. Feasting should never result in gluttony, intoxication or debauchery, while fasting should not be done for show, nor for proving to ourselves how admirably we keep our principles.
By fasting, we secretly give up something that is normally pleasing to us for love of Jesus, while empathizing with the poor who may be deprived of these goods.
The church still requires us to abstain from meat on Fridays, yet allows us to substitute this with a different sacrifice should we wish to. During Fridays of Lent, we are to abstain from meat regardless of other sacrifices. The essential thing is to practice dying to self on the day of the week that Jesus died for us.