Q: will my iPhoto Events be accessible on El Cap?

If you have an up-to-date version of iPhoto, then yes. If not you'll be able to access them via Photos.

Photos does not have events - when you migrate your events will be converted to albums and stored in a folder named iPhoto events - to continue you will need to manually simulate events by importing and then going to the last import album, selecting all and clicking the + sign and creating a new album

Info on how the transition works is here - How Photos handles content and metadata from iPhoto and Aperture - Apple Support - and it is worthwhile to read through this article and do the clean up and prep work before the transition - Notes on Migrating an iPhoto Library from iPhoto to Photos for Mac

LN

Moments in Photos are the new Events, i.e. groupings of photos sorted by date taken.

When the iPhoto Library was first migrated to Photos there is a folder created in the sidebar titled iPhoto Events and all migrated iPhoto Events (which are now Moments) are represented by an album in that folder.

To open the sidebar if it's not already open use the Option+Command+S key combination.

NOTE: It's been reported by several users that if the Event albums are moved out of the iPhoto Library folder in the sidebar they may disappear.  It's not widespread but several users have reported that issue.  Therefore, if you want to assure that you keep those Event albums don't move them outside the iPhoto Events folder.

There's a way to simulate events in Photos.

When new photos are imported into the Photos library go to the Last Import smart album, select all the photos and use the File ➙ New Album menu option or use the key combination Command+N.  Name it as desired.  It will appear just above the iPhoto Events folder where you can drag it into the iPhoto Events folder

 

When you click on the iPhoto Events folder you'll get a simulated iPhoto Events window.

Albums and smart albums can be sorted by Title, by Date with oldest first and by Date with newest first.