When running a calendar spread with calls, you’re selling and buying a call with the same strike price, but the call you buy will have a later expiration date than the call you sell. You’re taking advantage of accelerating time decay on the front-month (shorter-term) call as expiration approaches. Just before front-month expiration, you want to buy back the shorter-term call for next to nothing. At the same time, you will sell the back-month call to close your position. Ideally, the back-month call will still have significant time value.