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A calendar is a system of organising periods of time, typically days, weeks, months and years. A calendar business designs and prints calendars, this can include for commercial purposes (to resell), promotional purposes as well as specialised calendars such as sporting events and fixtures.
For making your own calendars you will require a computer with design software, a printing press, paper, and camera. Different business models with have different strategies here, example: people in the advertising calendar business often times outsource the printing.