GST returns (GSTR-1 and GSTR-3B)
The regular statements a registered business files: GSTR-1 lists its sales, GSTR-3B sums up and pays the tax.
GSTR-1 is the list of your sales invoices. It is what your business customers' input tax credit is built from, so filing it on time matters to them as well as to you. GSTR-3B is the summary: tax on sales, credit on purchases, and the payment of the difference.
How often you file them, and on which day, depends on how you are registered — the filing calendar works it out for your business.