Each December, Sephora mails gift cards. You may have gotten it via snail mail or email [[or, if you're me, you got it both methods]]. The card is good for $15 off a $50+ purchase at Sephora or online at Sephora’s website.
It is valid now through December 10, so you have a little bit of time to decide what to buy.
And, if you’re not going to use yours, you could always pass it along to your fave blogger who will put it to good use.