The Linux 'cal' command
Five things you should know about the Linux
calcommand. There are many, many, more features listed in the man page but these are the ones you should know.
The Unixread more
calcommand appeared in 1973 in what was called “Version 5 AT&T UNIX”. Linux and macOS (and all *BSD systems) have very different clones. Check the man page for details.