@chuso: For index Futures and interest rate futures there are indeed known rollover dates.
For commodity futures this is not the case, but most participants roll according to volume or open interest crossover.
If you roll a contract to a new delivery month, this may be a different crop with entirely different conditions for supply and demand. In case that the harvest in 2012 was not good, this does not mean that the harvest 2013 will be bad as well. Agriculturals are perishable goods, and if they are stocked until the expiry of the contract, you may well have a different product.
I do not think that there are any precise rollover dates for agriculturals. Volume tends to shift to the new front month contract 1-10 days prior to first notice day.
For metals the situation is much easier.
For the gold contract there is a known first notice date. For the current front month GC 02-13, the first notice date is January 31st. Gold futures typically roll 1 or 2 days prior to first notice day.
The metal does not perish, it is basically a financial contract and the rollover gap does not depend on the day on which you roll, pretty much as you would observe for other financial futures.
Silver and copper futures also roll 1 or 2 days prior to first notice date, so all you need to do, is to look up those dates and roll accordingly.
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CME does not post an official rollover date for energy and metal contracts like it does for stock index futures. Generally trading volume shifts to the next active futures as mentioned above: a day or two before first notice day. You need to watch where the volume is and where the best bid/offer spread is before first notice day to decide when to switch.
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