The stronger the memory, the better. It should have more power that way.

[He's already got an idea of a memory that might work, but he holds his tongue-- this should be Day's decision, after all. It needs to be a memory that holds particular meaning to him.]

And given that magic here comes from the heart, I would think a memory that has a significant positive association with it would be the best choice.

[Now it's his turn to glance over at Ginger for agreement.]