Ann: WWI Investor Webinar, page-134

On the third Thursday of the month at the end of the quarter there is a delayed opening on the stock market.
On this day monthly options and quarterly futures contracts expire at the end of the opening. So there is usually a 20 minute opening on this day. So contract prices are calculated on the open, for example if you have an XJO (option trade on the top 200) you get the opening price on each of those top 200 share prices. So for example BHP might open up $1.40 by the time WBC opens 15 minutes later BHP might be back to the previous nights close, but your option price is calculated on the opening price so the index might open this morning up or down 40pts but the calculated open on all the top 200 could be 15-20 pts higher or lower. This can make a big difference to expiry on options and futures contracts.
Hence the delayed opening this morning today is the third Thursday on the month in the last month of the current qtr. it’s usually a 20 minute opening on this day.