How to find your old iPhoto and Aperture events, stacks and projects in Photos for OS X
Photos may be a great upgrade for iPhoto and casual Aperture users but, after you migrate, you might need some help finding your old pictures.
Anything previously organized into events in iPhoto or stacks, projects, or events in Aperture gets brought over to Photos during setup, of course. The stacks, projects, or events themselves, however, are converted into folders. That means they're all the same now, but it also means they're really easy to find.
Full story from the iMore Blog...