calendar / calender are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
calendar: (noun) a system of timekeeping that defines the beginning and length and divisions of the year. (noun) a list or register of events (appointments or social events or court cases etc). (noun) a tabular array of the days (usually for one year). (verb) enter into a calendar.
calender: (noun) a machine that smooths or glazes paper or cloth by pressing it between plates or passing it through rollers. (verb) press between rollers or plates so as to smooth, glaze, or thin into sheets.
calendar: k·AE·l·ah·n·d·er
calender: NA
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)