Why is Maundy Thursday called Maundy Thursday?
This is the easy one. Maundy is the English derivation of the Latin mandamus, meaning commandment, as in "A new commandment I give unto you: Love one another as I have loved you."
Why is Good Friday called Good Friday?
Here, I'll draw from the 1913 Catholic Encyclopedia:
From the earliest times the Christians kept every Friday as a feast day; and the obvious reasons for those usages explain why Easter is the Sunday par excellence, and why the Friday which marks the anniversary of Christ's death came to be called the Great or the Holy or the Good Friday. The origin of the term Good is not clear. Some say it is from "God's Friday" (Gottes Freitag); others maintain that it is from the German Gute Freitag, and not specially English. Sometimes, too, the day was called Long Friday by the Anglo-Saxons; so today in Denmark.