I found this
page on the S&P site regarding methodology. Near the bottom are a bunch of spreadsheets, one of which is the rebalancing
calendar for equity indexes. Here's a snapshot:
View attachment 583673
If I'm reading that right, the actual file with the new balances for the 9/18 rebalancing is sent to SPG's customers after the close on 9/4. I assume any related additions/removals would be announced at the same time.
Note that announcements related to the previous rebalancing (6/19) were made on 6/12, the date listed for the proforma file in that calendar. Note also that both the June and December windows are one week between proforma and effective dates, but the window for September is two weeks.