Why Go Back To The Very Day Biff Received The Almanac?
And risk bumping into past Marty and Doc?
Why not just go to the day after?
Easy.
They wanted to retrieve it as quickly as possible. It was vital to keep track of the book.They didn't know his older self was going to tell his younger self to keep it on him everywhere he went. He could've just hidden it away in his house for a few years until he was old enough which he probably ended up doing eventually because old Biff tells him to get a safe.