The calendar is installed by default in Debian, however if you are running Ubuntu or another Ubuntu based distribution you may need to install it by entering the following command
sudo apt-get install ncal
To display a calendar in the terminal enter the following command
cal
To display a specific month we use the -m switch followed by the month
cal -m December
To display a calendar for a specific year we use the -y switch followed by the year
cal -y 2020
To add (x) months after the current month we use the -a switch followed by the number of months