or it is the first day that the stock becomes excluded.
Thank you! 
Hi Xl2940,
It is the last day when the stock is included in the S&P500.
Thank you.
Previously i found stock ‘CTX.US Equity’ 's last day is 2009-08-17, while in the inclusion table, the value for it is 0
This is done to signal that no weights should be allocated to the stock on the last date, as allocations are done the day after.
Got it! Thank you:)