Purchase Day Book:
The Purchase Day Book is a subsidiary book of accounting that records all credit purchases of goods and services made by a business. It is a chronological record of all purchases made on credit, typically organized by date.

Posting to the Debit of Purchases A/c:
When the total of the Purchase Day Book is posted periodically, it is posted to the debit side of the Purchases Account. The Purchases Account is a nominal account that is used to record the total cost of goods purchased by a business during a specific period.

Reason for Posting to the Debit of Purchases A/c:
1. Nature of Purchases Account:
The Purchases Account is a nominal account, and according to the rules of double-entry bookkeeping, all expenses and losses are recorded on the debit side of nominal accounts. Since the total of the Purchase Day Book represents the total cost of goods purchased, it is considered an expense and therefore posted to the debit side of the Purchases Account.

2. Increase in Purchases:
When goods are purchased on credit, it increases the purchases of a business. By posting the total of the Purchase Day Book to the debit side of the Purchases Account, it reflects the increase in purchases and helps in calculating the cost of goods sold.

3. Accurate Financial Statements:
Posting the total of the Purchase Day Book to the debit side of the Purchases Account ensures that the expenses are accurately recorded in the financial statements. It helps in determining the cost of goods sold, which is an important component in calculating the gross profit of a business.

4. Matching Principle:
The matching principle states that expenses should be recognized in the same period as the revenue they help generate. By posting the total of the Purchase Day Book to the debit side of the Purchases Account, the expenses related to the purchases are matched with the revenue generated from the sale of those goods.

Conclusion:
The total of the Purchase Day Book is posted periodically to the debit of the Purchases Account because it represents the total cost of goods purchased, and the Purchases Account is a nominal account used to record expenses. This posting ensures accurate financial statements and follows the matching principle.