This is in regards to the Reading(s) before the Gospel in the ORDINARY FORM of the Mass of the Roman Rite. At the Sunday Mass, there are two Readings before the Gospel. One is taken from the Old Testament. The second is taken from the New Testament. At weekday Masses there is one Reading before the Gospel. Why is there one Reading before the Gospel at weekday Masses and two Readings before the Gospel at the Sunday Mass?
The two readings on Sundays and HDOs express the solemnity of the occasion. Weekday Masses aren’t solemn, so there is only one reading.
For centuries there was only the epistle reading and the gospel reading. Adding a second reading before the gospel is a sort of extra added onto masses on Sundays and important feast days so that the people get more of the Bible during mass, but it would be burdensome to add it to weekday masses too.