Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Gcal can be used for a lot of calendar related questions. A zillion calendar options (did you wonder what the old armenic months are?), more than you probably know about. Of course, if you are not a calendar freak, this is not the main selling point. Gcal works (and is intended to) as a diary, and appointment planner. It can also be used to determine sunset or sunshine in your local coordinates, and if you are an amateur astronomer, it can give you the twilight times. The same holds for data for the Moon, it can give you the moon phase, for instance.
Hey, its already the 21st century! Command line? I can hear you mumbling How come I have to use a command-line calendar?'Of course, if you are into really pretty UIs... Skip this article. This is more for the Unix/Linux user who just pops open a terminal and shouts gcal -f $mycalendar when he wants to know what today is due.