The words calendar, calender sound the same but have different meanings and spellings. Why do calendar, calender sound the same even though they are completely different words?
The answer is simple: calendar, calender are homophones of the English language.
Any of various systems of reckoning time in which the beginning, length, and divisions of a year are defined.
A table showing the months, weeks, and days in at least one specific year.
A schedule of events.
An ordered list of matters to be considered: a calendar of court cases; the bills on a legislative calendar.
A machine in which paper or cloth is made smooth and glossy by being pressed through rollers.
To press (paper or cloth) in the rollers of such a machine.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.