The GSTR-3B due date for May 2021 remains the 20th of June. However, specific conditions apply regarding late fees and interest charges based on your business turnover. For businesses with a turnover under Rs. 5 Crores, there's a 30-day waiver on late fees and nil interest for the first 15 days, followed by 9% interest for the next 15 days, and 18% thereafter.