The last trading day of stock index futures is the third Friday of the contract delivery month (postponed in case of legal holidays), that is, once a month.
Specific trading rules can be found in official website, China Financial Futures Exchange.
For example:
An investor holds a contract of 1804, so the third Friday in April of 20 18 is its last trading day. If it happens to be a legal holiday, it will be postponed to the next trading day. Note: the extension here is not extended to next Friday, but the next trading day.
The above is the interpretation of the last trading day of stock index futures!