A sales day book is a book of prime entry used to record credit sales of stock. It is a chronological record of all credit sales made by a business during a specific period, usually a day or a week. The book contains details of the sales, such as the name of the customer, the date of the sale, the invoice number, the amount of the sale, and any relevant notes. The purpose of the sales day book is to provide a complete record of all credit sales transactions that occur in the business, which will be used for subsequent accounting and financial reporting purposes.