The general principle is that the later addendum supersedes the previous, so if the newest addendum states closing on September 26th, that is when all parties who have signed it says it will happen.
If you are discussing possession, when you get keys, that will be either on closing if the contract calls for that and hasn't been amended or closing plus whatever changes have been agreed to.
Closing is when the home legally transfers ownership from seller to buyer, possession is when the buyer gets keys and can access. These dates can be the same or different by mutual agreement.
This can't be answered without seeing the whole contract.
You should as your agent, and/or the office designated broker.
Jean Bradford, ABR,GRI,CRS,CRB,SFR
John L. Scott