Settlement is the day on which the completion of the transaction is scheduled to occur.
You should negotiate a specific date for settlement before exchange if you need to make settlement more convenient to you
When you sign the contract you should pay attention to the date on which settlement is to complete.  If you later need to change that date you can ask the other party to agree however in most cases it is unlikely that they would agree to vary the contract.  Varying a contract should be undertaken in writing between the parties and extra legal fees will be incurred as a result.