An accounting period, in book keeping is the period with reference to which accounting books of any entity are prepared. 

It is the period for which books are balanced and the financial statements are prepared. Generally, the accounting period consists of 12 months. However the beginning of the accounting period differs according to the jurisdiction. For example, one entity may follow the regular calendar year, i.e. January to December as the accounting year, while another entity may follow April to March as the accounting period.

If a company prepares monthly financial statements, there needs to be adjusting entries as of the last day of every month to:

• Accrue expenses and liabilities that occurred but have not yet been recorded. Examples include maintenance, repairs, wages of hourly paid employees, utilities used, property taxes, interest, etc. 
• Record the depreciation for the 30 days of the month. 
• Adjust prepaid expenses for the amounts that have expired and to defer the expenses that have not expired as of the end of the month.