The calendar was invented to organize the days of the year for different purposes, mainly agriculture. The calendar symbol takes the shape of a paper calendar that are usually hung on the wall or put on a desk. The symbol is used in typing to indicate an upcoming event or memorial. It's also used widely to indicate time, schedules, observance, planing or dates.
The number displayed on the symbol may indicate a special occasion for the platform that uses a specific icon. For example, twitter uses 21 of Mars, 17 July is used by the Emojipedia because this is the day that it was first used this symbol.