You can look at any receipt you get from a cashier and it will tell you the expiration date. Also its good until the end of the month you bought it on. So if you got it in May 2018 it expires at the end of May 2019.
While membership is typically for one year, I usually get an extra 2 months for free when I signed up through work. Check a receipt or with the customer service desk to see what your expiration date would be.
Are you talking about a BJs membership card? You would have to go to a membership desk in store to find out if your card is still active.
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