If you have manual books of accounts:
​
As per the BIR:
​The registration of a new set of manual books of accounts shall only be at the time when the pages of the previously registered books have all been already exhausted, provided, that the portions pertaining to a particular year should be properly labeled or marked by taxpayer. This means that it is not necessary for a taxpayer to register/stamp a new set of manual books of accounts each and every year.

In other words, you only have to get new books stamped if you've exhausted the previous one. :)

You can read more about it here: https://www.bir.gov.ph/index.php/registration-requirements/secondary-registration/registration-of-book-of-accounts.html

​If you have loose leaf books of accounts:

You need to hard bind your loose leaf books for the year and have them stamped by the BIR 15 days after the calendar year ends. So technically, it's January 15. Don't forget to follow the format that was approved by the BIR when you received your Permit-To-Use (PTU) loose leaf books of accounts.

You'll also have to renew your registration (pay the renewal fee of BIR) before you have your loose leaf books of accounts stamped.