Unlike equity derivatives where the underlying stocks trade on the same exchange, in case of commodities the underlying doesn’t trade on the same exchange. Also each underlying in case of commodities may not have any co-relation with another.
For example gold has no relation with guar seeds. When designing a commodity future contract, exchanges take into factor what determines that contract, rather than trying to force expire all contracts on the same day. Also in case of contracts where international factors also have a bearing, expiry dates are fixed by keeping those in mind as well. (Gold, silver, crude, copper, etc).
The expiry dates for commodity contracts are set in accordance with the time & effort required to complete the delivery of the contract.
The expiry date of each commodity differs because of their individual demand, production, and usage.
Also, there is no standard mechanism to fix the expiry dates for commodities because it is not required. Different commodities require a different period to fulfill the contract.
Future contracts for gold and silver would have different expiry dates than say for agricultural products because the harvesting time for agricultural products differs vastly.
Equity options, on the other hand, expire on the third Friday of the expiration month if the exchange follows the American style. European style exchanges have decided that the contracts would expire on the last Thursday of the month.
In case the third Friday or last Thursday falls on the exchange holiday, the expiry date is preponed to the preceding trading day!
The third Friday or last Thursday also become the last day the equity can be traded. The trader’s intention regarding the equity should be conveyed to his broker on or before the third Friday or last Thursday of the expiration month.
Most of the contracts are either cash settled or through the purchase of similar contract that nullifies the existing contract.