A calendar (or time spread) exploits the fact that theta accelerates near expiration. The short front-month option decays faster than the long back-month option. It works best if the stock sits near the strike as the front month expires.
Calendars are long vega: a rise in implied-volatility helps, and a collapse hurts.
Example: stock at $50. Sell the 30-day $50 call for $1.50, buy the 60-day $50 call for $2.60. Net debit $1.10. If the stock is at $50 in 30 days, the short call expires worthless and the long call is still worth about $1.80.
Related: theta, vega, vertical-spread, expiration-date