To save def some work.....but def, correct me if I am wrong. One to two days before last trade. For example, last trade for the September contract will be Friday 27 Sept. Most people tend to roll on the Wednesday and the Thursday. Volume on the Friday will miniscule.
hendrix is correct, typically 2 days before the expiration. The future rolls/spreads get highly liquid and 1 point wide. thus trading either month going into expiration is fine. However, beware of trading the expiring month on expiration day as settlement is determined by the average price of the index (cash) taken at 5 minute intervals during the day.
for hang seng stock index futures and is the liquidity good and spreads narrow in this index? sorry ... I know I should look this up but I am lazy after a long week ...
the roll from one month to gets tight with size going into expiration. (right now with 2 weeks to go the market is -2 at -1 50up. next week it will be 500 or 1000 up). liquidity: is ok. However, the market can get very volatile. about 20K contracts have been trading as of late. it is one of those markets that you really need to watch to see if it fits your style.