I noticed that last Wednesday seemed to be the final day for trading the June e-mini's, that is with the typical volume. Thursday was a switch over to the September contract. This Thursday (June 20) is the last trading day for the June contract. How come the switch last Thursday? Why a week ahead of time? Is it always 1 week prior to last day to trade?
Rollover is always on the Thursday one week and a day before the Friday expiration of a contract ----- the third Friday of the calendar month. Hence rollover occurs on the second Thursday of the month, 8 days before the contract's expirartion, unless the expiration is on Friday the 15th, in which case rollover is on the first Thursday (the 7th) of the month. The 8 days provide time for institutions, especially, to unwind large positions at their leisure without having to dump or rollover huge orders within a brief period of time.