Good Friday is not a day of obligation.
On Good Friday, a Solemn Liturgical Service is conducted, usually in the afternoon. This is not a Mass, since no Consecration takes place. In former years, this service was known as the "Mass of the Pre-Sanctified" meaning that the Hosts for Communion were previously consecrated at Holy Thursday's Mass. The one reason I am familiar with why an actual Mass is not said this day is because Good Friday recalls the very day that the original Sacrifice of the Mass (the Sacrifice of Calvary) took place. So, in deference to that we simply recall the original event. On all other days, we have the unbloody renewal of that same sacrifice, known as the Mass. Also, Holy Thursday recalls the Last Supper, which was the first Mass, as it was the foreshadowing of the Sacrifice about to take place. Mel Gibson did an awesome job of illustrating this very point in his flashbacks in THE PASSION. So, this explains using Consecrated Hosts from the previous night.