Book closure is when companies do not handle adjustments to their register or any requests to transfer shares.
Book closure is also used as a cut-off date to determine which investors will receive corporate action benefit.
Investors pay close attention to the book closure date as it determines when they should sell their shares or how long they need to hold onto them to receive a corporate action benefit.
The publicly-traded companies' stock changes daily as investors buy and sell shares. Because of this, when a company declares it will pay a dividend or issue bonus or split stock, it must have a specific date set on which it will close its shareholder record book and commit to sending the dividend to all investors holding shares as of that date.