As Day One Classic is no longer available for purchase, it has been removed from the App Store.
If you've previously purchased Day One Classic (version 1.x), you can download it from your purchase history. You must be using the same Apple ID and be located in the same country as when you first downloaded Day One Classic.
Tap your account photo at the top right of the App Store > Purchased > Search for "Day One".
(macOS 10.13 and earlier)
Mac App Store > Purchased (tab at the top) > then scroll to find "Day One Classic".
(macOS 10.14 and later)
Click your profile at the bottom left. Scroll to find Day One Classic.
iPhone (iOS 10)
App Store > Updates > Purchased, then search for "Day One". (See video below)
iPad (iOS 10)
App Store > Purchased (tab at the bottom).