Explain what calendar spreads are used for

A calendar spread pairs options on the same underlying and strike with different expiries. In a long calendar the manager sells the near-dated option and buys the longer-dated one. The near option loses time value faster, so the spread profits if the price sits near the strike at the near expiry. Being net long vega, it also gains if implied volatility rises. A short calendar reverses this, suiting a view that a big move is imminent or that longer-dated implied volatility will fall. Calendars trade time decay, express views on the volatility term structure, and position around a known event falling between the expiries. The trap is thinking a long calendar is safe: a sharp move either way erodes it.