Agricultural products futures: the main contract is only changed in three months: 1, 5, 9.

Metal futures: the main contract is to change the month from month to month, but generally it will skip February and sometimes it will skip August. Usually, the main contract month is a contract that is two or three months away from the delivery month.

Stock index futures: the main contract month is the delivery month contract, and the main contracts of different kinds of futures are different, which is mainly related to the differences of different futures trading rules. For example, stock index futures allow individual investors to hold positions in the delivery month, while commodity futures generally do not, so the main contract of stock index futures can be the delivery month.