Zenity is an open source and a cross-platform application which displays GTK+ Dialog Boxes in command-line and using shell scripts. It allows to ask and present information to/from shell in Graphical Boxes. The application lets you create Graphical dialog boxes in command-line and makes the interaction between user and shell very easy.
To Display Calendar:
To display text dialog:
To create a info dialog:
To display warning dialog:
There are other alternatives, but nothing compares to the simplicity of Zenity, specially when you don’t need complex programming. Zenity, a tool you must have your hands on.