Gsimplecal is a lightweight calendar applet written in C++ using GTK.

It was intentionally made for use with tint2 panel in the openbox environment
to be launched upon clock click, but of course it will work without it. In
fact, binding the gsimplecal to some hotkey in you window manager will probably
make you happy. The thing is that when it is started it first shows up, when
you run it again it closes the running instance. In that way it is very easy to
integrate anywhere. No need to write some wrapper scripts or whatever.

Also, you can configure it to not only show the calendar, but also display
multiple clocks for different world timezones. Read the manual page for the
details (there is info about keyboard controls as well!).