IPA: //ˈdeɪt.bʊk//
KK: /ˈdeɪtˌbʊk/
A book or planner used to keep track of appointments, events, and important dates.
I always carry my datebook to remember my meetings and deadlines.
Datebook is formed from "date" (from Old French *date*, meaning a specific day or time) and "book" (from Old English *bōc*, meaning a written work or record). The word refers to a book used for recording dates or appointments.
Think of a 'date' as a specific day and a 'book' as a place to write things down — a datebook is where you keep track of your important days.