In and of itself, a calendar is a neutral position where you expect (hope?) the underlying will expire near the strike sold.
Some other possibilities include:
1) Using them as bullish or bearish strategies (buying them OTM),
2) Double calendars (what I call calendar straddles and strangles) which can be ratioed to add some bang for the buck
3) Volatility plays where you hope to collect more from an IV contraction of one leg vs that lost on the other leg, or vice versa.
4) Reverse Calendars for special situations
1) Standard application is to buy a long calendar with the goal of the underlying finishing near the strike, ending with a reduced cost far month long option. I think this is a fool's errand since who knows where an underlying will be a month from now? If one knew that, one would be Miss Cleo. I say fool's errand because the bane of calendars is a move away from the strike thereby forcing the legs toward parity. Since the long leg cost more, it loses more $$ and the position fails.
2) Double calendars are interesting and what strike and ratio you use (if any) will depend on IV skew (if any) expectation/prediction of movement in the underlying, expectation of IV change, and risk tolerance. You can tailor some interesting risk graphs from these.
3) This ties in with #2 and #4. For earnings plays with skew, you're getting more risk premium for selling the near month and that gives you more tolerance for an adverse move. But just because you're selling a more over valued near month, even with an IV collapse, the far month can still lose more $$ because it cost more to start with.
4) Reverse calendars and particularly double reverse calendars (aka double reverse straddles) are interesting for that rare situation where vols are way out of line in all months just before a news release as well as for earnings releases the last week before expiration where a contraction (hopefully a collapse) in vols and/or price movement moves the underlying away from the strike sold. In some situations you can ratio a calendar strangle, giving yourself some extra cushion if the underlying goes nowhere and some initial directional profit if it goes somewhere but of course, not too much direction since the extra short ratioed legs then come into play and begin to hurt you. Look for a chain here by IV Trader who introduced this strategy to a lot of us.
Every option strategy has potential surprises. IMHO, everything is a trade off of risk and reward and you have to find a balance in a position that you can swallow. Calendars are no different. There's always an area in the risk graph that costs you. By combining them, you can take some of the pain of a directional move out but that introduce some risk elsewhere. So it all depends on where you want your pain located
