Your bank debits the money before the scheduled eMandate transaction date to ensure the funds are available in your trading account on the specified date.
Why this happens
eMandate transfers take one working day to reflect in your trading account. Your bank debits the money during banking hours, but the National Payments Corporation of India (NPCI) credits Zerodha's bank accounts and confirms the transaction at the end of the day. Your bank therefore debits the funds at least one working day in advance so the money appears in your trading account on the transaction date.