It’s a very simple tool, built by Tijs Vrolix to show off his coding and design skills: Subscribe to messages from any Twitter user in any popular desktop or online calendaring application (iCal, Google Calendar, etc.). Those messages are then automatically added to the calendar, at the appropriate day and time.
Useful? I don’t know. It’s certainly useful to closely monitor/stalk people (or yourself). If you want to add my daily words of wisdom to your calendar, my page is here (which also includes a graph of total twitter usage by day.
Thanks to Robin Wauters for the tip.