If you'd like to join for the summer, simply go into your local store between now and June 7th and let them know you heard about the summer sign up for $15. For $15 you can be a member of Sam's Club for the duration of the summer. Would work great for those of you needing pool supplies etc that might be more expensive elsewhere.
I find that there's not much at Sam's that warrants my purchase of a membership, but I do enjoy the occasional stroll through the store to see what they have to offer. You will find a few things that are a good buy when compared to using the sales and coupons approach elsewhere, but not a lot in my experience.
Enjoy your freebie day... or your summer shopping there if you choose that option :)