Google Calender is free and you don't even have to download it, because it's web-based.
The poor man's calendar is a spreadsheet or word processing document. Those are free to make with OpenOffice.
If the above aren't satisfactory, you might consider to disclose your hidden requirements.
Kees
Need suggestions for free (safe) download of calendar to manage TO DO list.
Thanks
Ben

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic