all futures contracts have 2 important dates.
first notice and last trade.
first notice is when the exchange gives all open positions a chance to close out their trades, if they remain open, it shows they want delivery or deliver the goods.
last trade, is just that, the last day for that contract month to trade, if you don't clsoe your trade by this date, you will get assignment of your open trade. If it is a commodity, you'll get the goods dumped on your front lawn and if you are short, well then, guess you're gonna have to come up with the goods somehow.
without getting your mind too wrapped up about this, just compare trade volume between months. Once the back month starts to trade more than the front month, its about roll-over time. Join the rest of the party in the back month. You'll even notice it from the price action, its not as smooth in the front month.
last trade for index futures should be the 3rd friday every 3 months, triple witching as they call it, all three derivatives expire, index futures, stock options and index options.