You should schedule your eMandate transaction a few days before your stock SIP date because the bank confirmation of the eMandate transfer can be delayed. For example, if your stock SIP is scheduled for the 5th of every month, you should schedule your eMandate transaction for the 2nd or 3rd.
Your bank account is debited one working day before the eMandate scheduled date, and you receive SMS and email confirmations one day in advance.