happy: opportune. Later in the scene we learn that Romeo carries his dagger on his back (lines 203-204), in the same way that swords were sometimes carried in the middle ages.
This explains how the dagger comes into Juliet's hand at an opportune moment. As she kisses Romeo she embraces him, and in the embrace, just as she hears the voice of the watchman, her hand finds Romeo's dagger.
Image Source: Zelda Dungeon