After the event that on the shopping district, where Asuna and Kirito knows about that marriage has been cancelled, find Asuna and invite her to your party. Then go to the labyrinth zone (the last area before fighthing a boss) whatever the floor it is. When you got to the second floor of labyrinth zone there should be an event where Asuna is crying.
After that event go to the 79th floor town and noon (you must pay attention with the clock) and visit the church. If your time is right there must be a purple mark on the church. The church is the selection number two from below of the 79th floor city. If you don't made it on time, you can spend the time in an inn where is the third selection from below of the 79th floor town until noon.
After the event, the father will say that you must find the material from the ring somewhere at the 79th floor field. Find the mark and battle the boss. After that go back to church at noon too. The event should be end there. But still, the engagement ring quest doesn't end at the 81st floor you know.
For the 82nd floor event quest for engagement ring, visit the lake on the 82nd town at noon with Asuna. For the 84th floor event quest for engagement ring, visit the new home on the 83rd floor with Asuna. For the 86th floor event quest for engagement ring, go to the 86th floor dungeon with Asuna.