I am a big businessman or professional. Big means my sales are high (you know its Rs. 1 cr in a year, being a businessman) or professional receipts are high (here it is Rs.50 lacs a year being a professional), this high means I need to get my accounts audited. So I need more time to cook and present my dish more formally. There’s more time given yes, my last date to file my audit is September 30 every year. Its for tax so its called Tax Audit. Then after I file my audit, I get a months time to file my return, which is 31st October
Sometimes, due dates are extended by Govt to take care of any special circumstances.