Utah Gal -- you are right. Now I don't get what happened in my case, but after a bit of analysis I do see what I think I did. Apparently in my desperation when I first discovered we no longer had the Photo Stream Event appear automatically, I tried some things, and one of them worked so smoothly I didn't realize it was of my own doing.
So here is my recipe for doing what I did.
1) After opening Photos, select Albums
2) From the File Menu select "New Smart Album..." and a new dialog box will appear.
3) Give your Album a name -- for example, "June - 2015"
4) In the first pull down menu select "Date"
5) In the second pull down menu select "is in the range" and two date selection fields appear
6) Put in the range -- for example "6/1/2015" to "6/30/2015"
7) Automagically your photos will appear in your monthly Smart Albums if you are using Photo Stream.
You could create Smart Albums for the rest of year if you wish. You might want to do one for May 2015 to capture those also. I am hoping what is in the Smart Albums is permanent even after the photos are dropped from Photo Stream. They should because they appear in "Photos" which has them all.
So Utah Gal, I hope this helps you and others. Your post helped me figure this out.